The researchers were super specific about which ones they included in this review. They only used studies where penis size was measured by a health professional, because…people lie.
They also included only studies that had at least 50 participants who were all at least 17 years old. To be consistent, they looked only at studies where flaccid and erect penis length was measured from the root to the tip (apparently there are other ways of measuring?) and penis circumference (girth) was measured at base or mid-shaft. And only studies published in the English language were included.
Studies were nixed if any participants could have had any congenital or acquired penis abnormality, erectile dysfunction, or if they were specifically seeking penis enlargement.