In an article published earlier this month, Ettman et al. (Ettman
and colleagues) estimated the prevalence of depression symptoms among US adults before and during the COVID-19 pandemic. (You can – but you do not have to – find the article here.) Data from before the pandemic were collected in 2017-2018 from a nationally representative sample of people. Data during the pandemic were collected in March-April 2020 from a different nationally representative sample of people (different people from the 2017-2018 survey). For both surveys, depression symptoms were assessed using the Patient Health Questionnaire-9, a clinically validated questionnaire with scores that could range from 0 to 20. Participants were classified as having depression symptoms if they had a score of 10 or greater.
a. Why is it appropriate to calculate prevalence rather than incidence of depression symptoms given the data from this study?