Regional differences in average penile length

https://www.myintimacy.com/guides/average-penis-size/ why do some of the sources for the claims of average penis size, length or girth, as well as mention of sample sizes, seem to not exist?

Many popular guides and listicles cite “” figures without clear sourcing because the underlying literature is methodologically messy: studies mix , use different measurement techniques (flaccid, stret...