Hair calibre is a major determinant of visible coverage. A person can have a reasonable number of hairs but still look thin if the hairs are very fine. In androgenetic alopecia, calibre gradually reduces as follicles miniaturise. In treatment studies, increased calibre can be an important sign of response, sometimes before dramatic density changes are visible. Calibre should be assessed consistently because hair products, lighting and magnification can affect perception.
DermNet trichoscopy of generalised non-cicatricial hair loss: https://dermnetnz.org/topics/trichoscopy-of-generalised-noncicatricial-hair-loss ; Anastassakis, Androgenetic Alopecia From A to Z, Vol. 1: https://doi.org/10.1007/978-3-030-76111-0