Fig. 1.

Schematic showing recruitment-based biases in a hypothetical serosurvey. This figure shows a hypothetical serosurvey that aims to measure the underlying seroprevalence in the entire population of a geographic region and performs recruitment among shoppers at a grocery store. Outline color represents prior SARS-CoV-2 infection status (red for prior infection, blue for no prior infection). Ascertainment bias occurs because (1) individuals recruited at the grocery store are likely at slightly higher risk of COVID-19 than average (since individuals who are isolated at home and rarely grocery shop are less likely to be sampled), and (2) nursing home residents and other populations in congregate settings are excluded from the sample. Among individuals present at the grocery store, volunteer bias occurs when individuals who believe they have been infected participate in order to receive testing. Meanwhile, individuals who do not think they have been infected may avoid testing