The GAD-7 and the PHQ-8 exhibit the same mathematical pattern of item responses in the general population: analysis of data from the National Health Interview Survey

Background Recent studies have shown that, among the general population, responses to depression-rating scales follow a common mathematical pattern. However, the mathematical pattern among responses to the items of the Generalized Anxiety Disorder-7 (GAD-7) is currently unknown. The present study investigated whether item responses to the GAD-7, when administered to the general population, follow the same mathematical distribution as those of depression-rating scales. Methods We used data from the 2019 National Health Interview Survey (31,997 individuals), which is a nationwide survey of adults conducted annually in the United States. The patterns of item responses to the GAD-7 and the Patient Health Questionnaire-8 (PHQ-8), respectively, were analyzed inductively. Results For all GAD-7 items, the frequency distribution for each response option (“not at all,” “several days,” “more than half the days,” and “nearly every day,” respectively) was positively skewed. Line charts representing the responses to each GAD-7 item all crossed at a single point between “not at all” and “several days” and, on a logarithmic scale, showed a parallel pattern from “several days” to “nearly every day.” This mathematical pattern among the item responses was identical to that of the PHQ-8. This characteristic pattern of the item responses developed because the values for the “more than half the days” to “several days” ratio were similar across all items, as were the values for the “nearly every day” to “more than half the days” ratio. Conclusions Our results suggest that the symptom criteria of generalized anxiety disorder and major depression have a common distribution pattern in the general population.