"Ascertainment" refers to the act of finding out or discovering information or facts about something. It involves obtaining knowledge or understanding on a certain subject. Full definition
The effect of study size, age groups at outcome measurement (comparing those aged 16 — 30 y with those aged ≥ 50 y), year of birth, the method of ascertainment of infant feeding status (whether contemporary or recalled over a period of ≥ 5 y) was examined by using meta - regression and sensitivity analysis. (academic.oup.com)
What is unclear is whether that abundance reflects the frequency with which co-option occurs in nature or whether it is the result of ascertainment bias (systematic sampling errors introduced by how biologists study evo - devo). (scientificamerican.com)
Correcting for ascertainment biases when analyzing SNP data: applications to the estimation of linkage disequilibrium. (nielsenlab.org)