Is it true that vellus hairs on any given area of scalp are relatively normal for a healthy scalp?

This a very crude cut off for diagnosing AGA but no one does not “normally “ find 20 % everywhere. Normally it’s closer to 10 % but the cut off of 20 % variation of hair diameter was set to make things easy for people.

In 2009, Rakowska et al. proposed 3 major and 3 minor diagnostic dermoscopic with a high specificity to diagnose AGA.

The major criteria were : more than four yellow dots in four fields decreased average shaft thickness and more than 10% of thin hairs (;0.03 mm) in the frontal area.

You can see the cut off here was 10 %

The more accurate the methods one uses to evaluate androgenetic alopecia the lower the cut off can be.

20 % is a cut off that is very generous and unfortunately if we all stick with that we will miss properly diagnosing alot of early staged AGA.