Young people with gender dysphoria have high rates of mental health issues which makes conducting ethical research trials complex.
Once upon a time, a group of disheartened scientists found their tearoom bereft of teaspoons. They explored the problem with a longitudinal study design.
Any field that collects and analyzes data relies on statistical techniques to make sense of it all. Modern, more accurate methods should supplant the old ways... but in many cases, they haven't yet.