Archive
Everything published here
205 pieces, newest first.
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Adding up the errors
Every reading carries several small errors from different sources; an error budget lists them, sizes them and shows which one to fix first.
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Meta-analysis statistics, start to finish
The statistical machinery behind a pooled size figure: weights, fixed versus random effects, heterogeneity and the intervals that follow.
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Reading a size study critically
A step-by-step guide to judging any penis-size paper or press story: who was measured, how, by whom, what was pooled and what the number can support.
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Repeatability and agreement, start to finish
What it means for a measurement to repeat, how agreement between two readers or two instruments is quantified, and how to test your own.
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Sitting down changes the bone landmark
A note on how a seated posture can fold the lower abdomen and shift where the ruler meets the bone.
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Which ruler
A ruler for length needs a true zero at the end, clear millimetre marks, enough length and a flat edge; most school rulers pass and a few common ones fail.
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Marks and cleaning
Marking a tape with pen or cleaning it with solvent can lift printed graduations; use a clip or a thread for the overlap mark and wipe with water only.
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The ratio
Dividing the mean length by the mean girth does not give the mean ratio, and the ratio people quote from the averages is not the ratio most men have.
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Full rigidity, or the number is not comparable
A partial erection reads shorter and narrower than a full one, and "mostly erect" is not a state that yields the same number twice.
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Putting a plus-minus on it
With a handful of readings you can state a range honestly - half the spread, or the standard deviation if you have five or more - and you should.
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Small samples, wide intervals
A study of a few dozen men can report a mean a centimetre from the pooled estimate purely by sampling; its confidence interval says so if you read it.
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Digital callipers
A digital calliper displays hundredths of a millimetre; on compressible tissue the last two digits are noise and the display is a distraction.
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What was cited before
Before the meta-analysis, single studies served as the reference, each with its own method and sample; knowing them explains older figures still in circulation.
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What went into the pool
The meta-analysis included only studies where a health professional took the measurement, and that single rule shapes every number it reports.
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Not alone in the room
Company changes arousal state and adds an audience effect on rounding; either measure alone or have the partner measure, not both.
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Measuring from a photo
Perspective, lens distortion and the reference object's plane all corrupt a length taken from a photograph, even with a ruler in the frame.
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The kit list is two items you own
A rigid ruler and a strip of paper reach the same accuracy as anything sold for the purpose; the instrument is not where the accuracy is lost.
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Heaping
Self-reported figures cluster at whole and half units far more than measured ones do, and the pattern itself is evidence of rounding, mostly upward.
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What normal range means
In a results table, normal range means the middle 95 percent of the sample; it is a description of a distribution, not a judgement, and papers use it that way.
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Seeing through the ruler
A clear ruler lets you look straight down through the scale at the tip, which is the geometry that removes parallax; opaque rulers force a sideways sight line.
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Nominal width
Condom sizes are given as nominal width, the flat width of the sheath, which is roughly half the circumference; here is the conversion and the site to measure for it.
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Flaccid and erect percentiles
A man's flaccid and erect figures usually land at different percentiles, because the two distributions are shaped differently and flaccid carries more noise.
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A centimetre near the middle
Because the middle of the distribution is crowded, a centimetre of measurement error near the mean moves a percentile far more than the same centimetre in the tails.
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When pi lies
A shaft cross-section is closer to an ellipse than a circle, so one diameter times pi under-reads girth; two diameters and Ramanujan's approximation do better.
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What the forums get wrong
Forum measuring guides agree with the clinical method on bone-pressing and diverge on nearly everything else; a side-by-side of the two.
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Printing a ruler
Printers scale pages by default; a printed tape is only a tape if you print at 100 percent and check it against a known length before trusting it.
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Flaccid and erect are loosely related
Flaccid length predicts erect length only weakly at the individual level; the literature reports a correlation, not a rule, and stretched length does better.
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The bendy ruler
A flexible curve ruler follows an arc and reads its length, which is precisely the number the literature does not use; useful for curiosity, not for comparison.
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Forest plot
A forest plot lines up each study's estimate with its interval and the pooled diamond underneath; it is how you see heterogeneity at a glance.
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Meta-analysis
A meta-analysis pools published studies into one estimate; it inherits their methods and their biases, and it cannot be better than the studies it pools.
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Where zero sits
Many rulers start their scale a few millimetres in from the end; press that end to bone and every reading is short by the offset.
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Z-score to percentile
Subtract the mean, divide by the SD, look up the normal table - three steps take a measurement to a percentile, and each step carries an assumption.
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How the marks are made
Printed markings wear off and can be printed slightly off scale; engraved marks on a steel rule are the more trustworthy of the two, and cost the same.
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The end tab
Sewing tapes often start their scale inside a metal tab, and using the tab as zero shifts every reading; find the printed zero.
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Two kinds of spread
Spread across three readings in one session is technique; spread across sessions on different days is state. Telling them apart tells you what to fix.
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Who gets measured
Most measured data come from men attending urology or sexual-health clinics for other reasons, and that sample differs from the population in ways that matter.
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The online survey problem
Anonymous online surveys combine self-selection, self-measurement and no verification, and the figures they produce sit above every measured study for those reasons.
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The calculator checklist
A percentile calculator worth using names its reference study, the state and method it assumes, and shows a range for your uncertainty rather than one number.
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Same time of day
Repeat readings taken at the same time of day remove one variable you cannot otherwise account for; it costs nothing and tightens the spread.
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When the skin moves
Shaft skin slides over the underlying tissue, so a tape can bunch or drag it and change the circumference it traces; wrap without dragging.
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Why the numbers differ between papers
Two clinician-measured studies can report means a centimetre apart because of state, method, population and instrument, before anatomy is involved at all.
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BPEL
Bone-pressed erect length: the ruler stops on the pubic bone, the shaft is fully erect, the reading is taken at the tip along the top.
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Millimetres or sixteenths
A millimetre scale resolves finer than sixteenths of an inch and is easier to interpolate; if your ruler has both, read the metric side.
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The stop ruler
A ruler with a perpendicular stop at zero gives a repeatable seat against the bone and removes the ruler-slid-past-zero error; a stop can be improvised.
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Three places to take girth
Girth at the base, at midshaft and around the glans are three different numbers; the convention is midshaft and the other two need labelling.
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Mark first
A small skin-safe pen mark at the girth site makes the next reading land on the same spot; without it, site drift is a hidden error.
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Two scales on one tape
Dual scales exist for the sewing market, not for accuracy; they cost nothing if you commit to one edge, and a factor of 2.54 if you do not.
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AR measure apps
Phone AR measuring tools resolve to roughly a centimetre on hard, well-lit, planar objects and worse on anything else; that is not a measuring instrument for this.
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Instrument drift
Tapes stretch, ruler ends wear, printed scales fade; an instrument checked a year ago is not the instrument you hold now, so re-check before a careful session.
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The builder's tape
A steel retractable tape cannot wrap a curve, its hook has deliberate slack, and its curved blade cannot lie along a shaft; it is the wrong instrument for both measurements.
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If you buy a tape
If you do buy a tape: fibreglass, narrow, printed zero at the very end, millimetre graduations on both faces, and check it against a ruler when it arrives.
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Measuring a curve
Published lengths are straight-line distances from bone to tip; following the curve with a tape gives a longer number that compares to nothing.
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Bone-pressed in the literature
Not every measured study pressed to the bone, and the pooled figures mix both; a reader placing a bone-pressed reading on them should know that.
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How often
Once you have a well-taken figure with an uncertainty, remeasuring reveals noise rather than information; there is no reason to repeat it often.
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How to read the histogram
Bin width, sample size and axis choice all change how a histogram looks; here is how to read one without being led by its shape.
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Fewer girth figures
Fewer studies measured girth, fewer still measured it erect, and the site was not always stated, so the girth pool is thinner than the length pool.
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Converting units
Convert once, from the raw reading, using 2.54 exactly, and round only at the end; converting a rounded figure and rounding again drifts by more than you expect.
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Is it a bell curve
The nomograms assume a normal distribution; the measured data are close to it, and where they are not, the tails are the part to distrust.
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A large sample tightens the mean, not the spread
A big sample pins down the average precisely; it does nothing to narrow the range of individuals, which is set by the standard deviation.
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The pi tape
Engineers' pi tapes are graduated so a wrap reads diameter directly; on a non-circular section they read an equivalent diameter, which is a reminder of what any conversion assumes.
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Instrument terms
Graduation, least count, zero error, tab, bevel, vernier, nominal width, pi tape: the instrument vocabulary in one place, one line each, linked to the post that explains it.
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Reading the tape edge
Read where the tape's zero crosses the same edge of the tape further round; reading across the tape's width to the other edge adds or drops a diagonal.
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Age
The pooled studies are of adult men across a wide age range; what they report about age is limited, and this post says only what the data support.
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The six-inch number
The round figure that everyone repeats traces back to mid-century self-report and has survived every measured study that came in below it.
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Order of operations
Take length first because the girth site is defined from it, and because a tape wrap can change state; the sequence is small but it is part of the protocol.
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Which average
Published size figures are means; the median and mode sit close because the distribution is nearly symmetric, but they are not the same number.
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Ruler material
Material barely affects accuracy at this scale; what matters is that the ruler is stiff, its end is square and its markings are legible.
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Stretched versus erect
The literature treats stretched flaccid length as an approximation of erect length; here is what that relationship is and where it breaks down.
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Volume from length and girth
Volume can be estimated from length and girth with a cylinder formula, but girth enters squared, so its small error becomes a large one in the result.
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NBPEL
Non-bone-pressed erect length: the ruler rests on the skin at the base, so whatever length is buried in the tissue in front of the bone goes uncounted.
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The fat pad, and the number it hides
The tissue in front of the pubic bone varies between people and within one person over time, which is the whole reason a bone-pressed reading exists.
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Erect or flaccid girth
Flaccid girth is even less stable than flaccid length; the literature's usable girth figures are erect, and that is the one to record.
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False precision
A converted figure with two decimals carries the precision of the original, not of the decimals; quoting it finer than it was measured is a small dishonesty.
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Test your instruments on something else first
Measure a jar or a bottle with your tape, a string and a paper strip before measuring yourself; the spread you get is your instrument error, isolated from technique.
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The bevelled edge
A ruler whose scale edge is bevelled brings the markings down to the surface being measured and cuts the parallax between mark and tip.
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Which tape
Old cloth tapes stretch, vinyl tapes stretch less, fibreglass-reinforced tapes barely stretch; the difference is a few millimetres over a girth and it accumulates.
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The girth data
Veale 2015 reports flaccid and erect girth with a tighter spread than length; here are the figures, the sample they come from and what the tighter SD implies.
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The condom-fit literature
Studies run to size condoms measure erect girth and length self-reported at home, with a fitted product as the reward; they are a separate dataset from the clinical pool.
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Finding the landmark
Bone-pressed means the ruler stops on bone at the base, above the shaft; here is how to find that spot and know you have it.
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Precise, accurate, or neither
Three tight readings can all be wrong the same way, and three scattered readings can average to the truth; the two problems have different fixes.
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How hard to press
Bone-pressed is a stop, not a force; the reading is repeatable when the ruler stops on bone every time, and it wanders when pressure varies.
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The phone as a ruler
An on-screen ruler is only as accurate as the app's knowledge of your screen's pixel density; check it against a card before trusting it, and it cannot press to bone.
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What the top percentile means in centimetres
Under the normal assumption the 99th percentile sits about 2.33 SD above the mean; the Veale figures show how far that is and how few claims survive it.
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The measurement card
Every rule that matters, on one card: state, landmark, line, angle, site, tension, three readings, median, method line.
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Self-tensioning tapes
Body-measurement tapes with a spring or lock apply the same tension every time, which removes the biggest girth variable a person introduces by hand.
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Standard deviation
The SD is the typical distance from the mean, and it is the second number you need before a mean tells you anything about where a figure sits.
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Paper tapes
Clinical paper tapes are cheap, do not stretch, and tear; they are close to ideal for a single girth reading and poor for repeated ones.
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Citing the paper
The reference is Veale, Miles, Bramley, Muir and Hodsoll, BJU International 2015; here is how to cite it and which figures to attribute to it.
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Zero error
Zero error is what an instrument reads when it should read nothing - a worn ruler end, a tape tab, a calliper that will not close - added to everything.
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Flaccid spread
Across the pooled studies, flaccid length has the largest relative standard deviation of any state, which is the data-side echo of how much it wanders.
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Hair in the way
Hair at the base can stop the ruler short of the bone by a few millimetres and does so inconsistently; part it or trim it before measuring.
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When only a tape is to hand
A flexible tape used for length follows the shaft's contour and bends into the fat pad, and both effects add length; if you must, here is how to limit them.
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What a calliper reads
A calliper gives a width across, and girth is a circumference; multiplying by pi is only right if the cross-section is a circle, and it is not.
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What to write down
A reading without its conditions is not data; the log needs date, state, method, site, instrument and all three raw readings, not just the answer.
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When the ruler cannot lie flat
A rigid ruler on a curved shaft touches at two points and lifts between them; the reading is still the chord, but the tip needs sighting square to the ruler.
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Shoe size
The shoe-size belief has been tested directly and found no useful relationship; here is what was measured, and why the belief survives anyway.
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What to round to
Report to the resolution your instrument and technique can support - a millimetre or an eighth of an inch - and never finer than the spread of your readings.
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Two calculators, two answers
Online percentile calculators differ because they draw on different studies, different states and sometimes self-report; the disagreement is in the inputs, not the arithmetic.
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The jaws press
Calliper jaws close on tissue and compress it, and the reading depends on how firmly they close; there is no natural stop, so it is not repeatable.
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How many readings
Three readings in one session, and three sessions on separate days, is the practical minimum that lets you see the spread and take a central value.
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One method change, one shifted mean
Switching a protocol from non-bone-pressed to bone-pressed, or from stretched to erect, moves an entire study's mean by an amount that dwarfs its confidence interval.
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Ethnicity
Veale 2015 looked for ethnic differences and concluded the pooled data could not support claims either way; that absence of evidence is the honest finding.
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Marks from the first reading
A tape or ruler pressed hard leaves a temporary indentation, and a second reading taken on it lands low; wait a moment or move the site.
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The tape has thickness
A tape's markings sit on its outer surface, one thickness out from the skin, so a wrapped tape reads slightly long by two pi times its thickness.
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Tapes with a display
A digital readout on a flexible tape adds a decimal and removes nothing: the tape still stretches, still needs to be square, and still reads where you tension it.
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Keep the tape square to the shaft
A tape wrapped at an angle traces an ellipse rather than a circle and always reads high; perpendicular to the axis is the specification.
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The first reading
The first careful reading most people take is the highest they will get, because everything from pressure to angle is unconsciously chosen and later readings regress.
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Three states, three averages
A single table of the Veale means and SDs by state, so the three figures that get confused for each other sit next to each other with their spreads.
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CI or SD
A confidence interval bounds the mean; a standard deviation describes the individuals. Papers report both and readers routinely swap them.
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What the clinics measured with
The pooled studies used ordinary rigid rulers and flexible tapes, sometimes a calliper for width; nothing exotic, which is why the method is reproducible at home.
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Fewer erect measurements
Far fewer men in the pooled data were measured erect than flaccid or stretched, which is why the erect figures carry wider uncertainty.
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Which way the ruler points
A ruler held level while the shaft points upward reads a projection, and a ruler tilted past the shaft reads long; the rule is parallel to the shaft axis.
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Systematic and random error
Random error averages out with repeats; systematic error does not, and most self-measurement error is systematic and points the same way.
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Reading a girth percentile
Because girth has a smaller standard deviation than length, a few millimetres of girth error move a percentile further than the same error in length would.
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How far to stretch
Stretched length is taken at the point where the tissue resists further extension, and inconsistent tension is the main reason the number moves.
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Two instruments, one measurement
Taking girth with a tape and again with a paper strip separates instrument error from technique error, because the two instruments do not share failure modes.
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Session length
A session should be short enough that erection state stays constant across the three readings; if it takes long enough to change, the readings are not comparable.
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Centimetres and scores are different objects
A length is a distance with an uncertainty; a rating is a judgement with a rubric. Confusing them is how a ruler ends up arguing with a score.
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Rounding bias
Most people round toward the next half-centimetre or half-inch when the reading is between marks; writing the raw mark before deciding is the fix.
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The studies have error bars too
Clinician measurement is better than self-measurement, not perfect; test-retest figures where reported set a floor under the precision of any pooled number.
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Which studies get published
Studies with striking findings are likelier to be published and cited; in this literature that favours unusual populations and surprising means.
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Wide tape, narrow tape
A wide tape averages over a tapering shaft and lies less flat around a curve; a narrow tape follows the site more precisely and is the better girth instrument.
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Height
Studies that looked for a link between height and length report at most a weak positive correlation - real in a large sample, useless for predicting any one person.
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How the tip meets the ruler
The reading is where the tip reaches on a ruler laid alongside; pushing the ruler into the glans, or holding it off the tip, shifts the mark by millimetres.
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Where the measurement ends
Length ends at the tip of the glans in a straight line from the base - not at the meatus, and not at the far point of a curve.
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Inside a study measurement
A participant in a clinical size study is measured standing, by a trained observer, with a rigid ruler and a tape, in a room kept warm, under a written protocol.
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The goniometer
A goniometer is a hinged protractor that reads the angle between two axes; it is the instrument the literature uses for curvature, and a phone protractor app approximates it.
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How fine a ruler really reads
A millimetre mark is itself a fraction of a millimetre wide, so the practical resolution of a ruler is about half a millimetre and no finer.
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Measuring curvature
Curvature is reported as an angle between the base axis and the distal axis; here is how it is measured and why it shortens the straight-line length reading.
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Measuring the fat pad itself
A skinfold calliper can put a number on the pre-pubic fat pad, which turns the bone-pressed versus non-bone-pressed gap from a mystery into a measured quantity.
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The ventral line
The underside runs from the scrotal junction and follows any upward curve, so it is longer than the dorsal line by an amount that varies with anatomy.
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Ruler length
A 15 cm ruler is too short for a comfortable margin; 20 or 30 cm avoids reading right at the end where the scale is often incomplete.
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The chart in the article
Percentile charts in magazine features are usually redrawn from a single study or a self-report survey without saying which; a reader should ask.
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How tight the tape should be
Snug means the tape touches skin all the way round without indenting it; tighter compresses tissue and reads low, looser reads high.
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Exclusion criteria
Measured studies exclude men with conditions that affect the measurement, which is correct for their purpose and also trims the tails of the reported distribution.
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Why flaccid readings wander
Flaccid length responds to temperature, arousal, time of day and recent activity, which is why the literature reports it with wide spread and few people should bother taking it.
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Volume by displacement
Displacement is the only direct way to measure volume, and it is impractical enough that nobody does it; the formula estimate is what circulates, with its own error.
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Bone-pressed or not: the two length numbers
Two legitimate length measurements exist, they differ by a variable amount, and only one of them is comparable to the published data.
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When two clinicians measure
Studies that had two observers measure the same men report good but not perfect agreement, which is the floor of uncertainty in even the best data.
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Percentile of what
A percentile is only meaningful relative to a named sample; the same figure is a different percentile against a clinic sample, a self-report survey or a single country.
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Reading the wrong side
Tapes with centimetres on one edge and inches on the other invite reading a number off the wrong scale, and the error is a factor of 2.54, not a millimetre.
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Standing or supine
Studies mostly measure standing; lying down changes the fat pad, the shaft angle and your view of the ruler, so posture belongs in the method line.
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The nomogram figures
Veale 2015 published nomograms for flaccid, stretched and erect length and for girth; each is a mean and a spread drawn as a curve, nothing more.
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The column after the mean
Every measured mean in a paper comes with an SD, SE or CI in the next column, and which one it is changes what the number means.
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Stretched length across papers
Stretched length depends on how hard the measurer pulls, and no two protocols standardise that identically, so it varies between studies more than erect length does.
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How rare the extremes are
Under a normal curve, each additional standard deviation from the mean cuts the remaining share sharply; the extremes people talk about are rarer than the talk implies.
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Two percentiles, one person
Length and girth are only moderately correlated, so most men sit at different percentiles on the two; the same percentile on both is the exception.
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Not every ruler is 1:1
Architects' and engineers' scale rules carry scales like 1:50 that look like centimetres and are not; a reading off one is wrong by the scale factor.
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What the authors say to be careful about
The paper's own limitations section names heterogeneity, clinic-based samples, uneven erect data and unverifiable ethnicity; a reader should carry them along with the means.
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Reading blind
Mark the tip position with a finger or clip, take the ruler away, then read it; separating the act of measuring from the act of reading removes expectation.
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Why length is taken along the top
Dorsal, lateral and ventral lines give three different lengths on the same shaft; the literature uses the top, and this is why.
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Nomogram
A nomogram is a chart that lets you read one quantity from another without arithmetic; here it maps a measurement to a percentile.
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When the reward depends on the number
Some self-measured studies gave participants a product sized to their reported figure; that design changes who takes part and how carefully they measure.
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Held horizontal, or left alone
Pressing an upward-pointing shaft down to horizontal for the ruler changes the reading; measure along the axis at its natural angle and say so.
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Why the papers use centimetres
Clinical studies report in centimetres to one decimal; inch figures in circulation are conversions, and every conversion is a chance for a rounding slip.
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Least count, resolution, accuracy
Least count is the smallest division, resolution is what you can distinguish, accuracy is closeness to true; an instrument can be fine on two and poor on the third.
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Why centimetres
The literature reports in centimetres, a millimetre is a cleaner resolution than a sixteenth, and one conversion at the end beats one at every step.
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The average people carry in their heads
Ask people what average is and they name a figure above what clinicians measure; self-report, media selection and viewing angle each push perception the same way.
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How Kinsey collected size data
Kinsey's figures were self-measured by participants and mailed back on cards, which makes them the founding example of the self-report problem.
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Mean or median
With three or five readings the median resists one bad reading; with many readings the mean uses more of the information. Use the median for small sets.
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Designing the honest comparison
The clean test of self-report inflation is the same men measured both ways under one protocol; here is what that design needs and what has come close.
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The school ruler
A cheap plastic ruler is typically within a fraction of a millimetre over 20 cm, finer than any technique error; its problems are the end and the flex.
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String and ruler
Wrap a string, mark the overlap, lay it on a ruler: it works, and its two errors are string stretch and the width of the mark you make.
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Which average did you mean
Flaccid, stretched and erect, bone-pressed or not, clinician-measured or self-reported - each has its own average, and the word alone names none of them.
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How the erect figures were taken
Erect measurements in the literature come from pharmacologically induced erections, self-stimulation in private, or self-report of an at-home reading; each is a different dataset.
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Finding midshaft
Midshaft girth is taken halfway between the base and the corona; here is how to find that point and keep it the same next time.
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You and the clinician read differently
A clinician and a self-measurer using the same ruler still diverge, for reasons of angle, motive and landmark rather than honesty.
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The 2D:4D literature
A handful of studies report a relationship between the second-to-fourth finger ratio and stretched length; it is a research curiosity, not a predictor.
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Rings and gauges
Graduated rings and slotted gauges give girth by which one fits, which is a coarse ordinal reading rather than a measurement; a tape is finer.
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One and two standard deviations
Under a normal curve about two-thirds of a sample lie within one SD of the mean and about 95 percent within two; applied to Veale, that gives real ranges.
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The other hand
Gripping the shaft to steady it while you wrap compresses tissue near the grip and can move the girth reading; hold at the base, away from the tape.
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Temperature and the reading
Cold shortens a flaccid reading noticeably and warmth lengthens it; erect readings are far less sensitive, which is one more reason to report erect.
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The method line
A single line - BP, erect, dorsal, rigid ruler, median of three - makes a figure comparable; without it the number is an anecdote.
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The card in your wallet
A standard payment card is 85.60 by 53.98 mm; it checks a ruler's scale, a tape's stretch and a printed scale in a few seconds.
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A second pair of hands
A partner reading the ruler removes your viewing-angle problem and adds an inter-observer one; here is how to brief them so the swap is a gain.
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Reading a plus-minus
A ยฑ is a statement about your readings, not a guarantee about the true value; it says the next careful reading will most likely fall inside.
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The world map of averages
The country-by-country averages that circulate online mix self-reported surveys with clinical studies of different methods, then rank them to one decimal place.
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One decimal in the table
A published mean is rounded to match its uncertainty, which is right for the data and wrong to treat as exact; the rounding is a statement about precision.
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Has the tape stretched
Lay the tape along a rigid ruler at 10 and 20 cm; a disagreement of a millimetre or more means it has stretched and every girth it gave read long.
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Why girth is not one number
The shaft tapers, thickens or bulges differently from person to person, so a girth reading is only meaningful with the site attached.
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Heterogeneity
When the studies in a pool differ more than chance allows, the pooled mean is an average of different things; Veale 2015 reports this and it should be read.
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Percentiles
A percentile is the share of the reference sample that measured below a figure; the 50th is the median, the 90th means nine in ten measured less.
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The folding rule
A folding rule's joints have play and its segments are thick, so its zero end and its edge both sit away from what you are measuring; use a one-piece ruler.
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Who volunteers
Men who agree to be measured for a study are not a random draw, and the direction of that selection is a standing caveat on every reported mean.
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A single reading
One careful reading gives you a figure to the nearest half-centimetre and nothing about your error; it is a start, not a result.
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Why self-reported figures run high
The gap between what people report and what gets measured is consistent, well documented, and mostly not dishonesty. Method explains more of it than motive.
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Laser measures, ultrasonic tapes, smart tapes
Laser distance meters, ultrasonic measurers and app-connected smart tapes each solve a different problem; none can take a bone-pressed length or a midshaft girth.
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The direction of the error
Most instrument errors here push one way: stretched tape reads long, tape thickness reads long, tab offset reads short, single-diameter calliper reads short. Knowing the sign helps.
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Looking down at the ruler
Reading a ruler from above and behind foreshortens the scale and shifts the tip mark; sighting square to the ruler removes a small but consistent bias.
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Bone-pressed length, start to finish
The definitive walkthrough of the length measurement the literature reports - landmark, pressure, line, tip, reading, repeats and how to write it down.
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The vernier
A vernier calliper reads to a tenth of a millimetre by aligning two scales; here is how to read one, and why that precision is wasted on soft tissue.
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The paper that made stretched length a proxy
A small 1996 study measured men flaccid, stretched and erect and found stretched length tracked erect; that finding is why clinics take stretched length today.
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Where the error comes from
Two readings of the same person twenty minutes apart can differ by more than a centimetre. Almost none of that is the ruler.
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How a size study is built
Recruitment, exclusion, measurer, instrument, state and reporting: each design choice moves the reported figure, and knowing them is how you read any study.
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Every instrument, and what each costs you
Rigid ruler, cloth tape, string, paper strip, calliper, photo, AR app: what each can measure, its typical error, and the failure it is prone to, in one table.
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Stretched flaccid length, and why the clinics take it
What stretched length is, why the literature leans on it, how it is taken to a resistance point, and how it sits against an erect reading.
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Everything you can wrap, and what each reads
Girth is a circumference, so the instrument must wrap or infer; here is every option, how each is read, and how each errs.
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Reading a nomogram without fooling yourself
What a percentile is, how a nomogram is built from a mean and a standard deviation, and how to place a measured figure on it with its uncertainty attached.
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The Veale meta-analysis, read properly
The paper the whole subject rests on: what it set out to do, what it pooled, what it reports, and where its own authors say to be careful.
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Verifying the thing you measure with
Every ruler, tape and calliper can be checked against a known object in a minute; this is how, for each type, and what to do when it fails.
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Girth, done the way the studies do it
Everything that decides a girth reading in one place - where on the shaft, how snug, tape perpendicular, what state, how to read and repeat.
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The measurement glossary
BPEL, NBPEL, stretched, midshaft girth, precision, systematic error, nomogram - each term in one or two sentences with a pointer to the post that explains it.
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Tape, ruler, or a strip of paper
The instrument is the part people optimise and the part that matters least. Here is what each one is genuinely good and bad at.
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What the published data actually says
One large meta-analysis, measurements taken by health professionals rather than self-reported. The numbers are lower and the distribution is narrower than almost anyone expects.
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How to measure the way the research does
Most of the disagreement online is people comparing numbers taken by different methods. There is only one that is comparable, and it takes two minutes.