Abstract

Most symptom measures either implicitly or explicitly distinguish between symptom presence and symptom severity. For example, item 2 on the PHQ-9, a commonly used measure of depressive symptoms, asks respondents to rate how much they have been 'feeling down, depressed, or hopeless.' The response options are 0 (Not at all), 1 (Several Days), 2 (More than half the days), and 3 (Nearly every day). Answering 0 indicates that the symptom is not present, and any response greater than 0 suggests the symptom is present. Higher values indicate higher severity of the symptom. Although the response options distinguish between symptom presence and severity, most users of the PHQ-9 score it by assuming that a 0 (i.e., no symptom), lack of symptoms, is the low end of the severity spectrum. However, clinically, there is often a distinction between experiencing symptoms and how severe any one of those symptoms is. Baldwin and Olsen (2023) developed a sequential item-response theory model that can be used to evaluate whether symptom presence and symptom severity should be separated or considered part of the same construct. We applied the sequential model to 3 datasets, a sample of 6242 participants, containing a variety of measures (e.g., Beck Depression Inventory- Second Edition, State Trait Anxiety Inventory, Penn State Worry Questionnaire). The results indicate that the Two-Part model has best overall fit out of the three models (Two-part, Extreme Response, Unique relationship), suggesting that symptom presence and severity should typically be considered distinct constructs. We discuss the implications for scoring and clinical use of symptom measures in light of our results.

Degree

College and Department

Family, Home, and Social Sciences; Psychology

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