measurement method and size data
How to measure, and what the number means
The method, done the way the published studies do it, plus what the distributions actually look like once you stop reading forum posts.
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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.