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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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.