The 39 symptoms of generalized anxiety disorder: Limited overlap among seven common GAD scales

There have been several content analyses made based on Fried's (2017) study on symptom content and overlap of items among widely used depression scales. There are to date no studies made on the item overlap of scales used in the generalized anxiety disorder (GAD) research. Therefore, the goal of this study was to investigate the symptom content and overlap of commonly used scales in the field of GAD.

A content analysis of seven scales used in GAD research was made to determine symptom content and overlap. Content overlap was measured with the Jaccard index ranging from 0 (no overlap) to 1 (complete overlap). For each scale rates of idiosyncratic symptoms, compound vs specific symptoms and percentage of GAD core symptoms were calculated.

We included seven scales with a total of 101 items which resulted in 39 disparate symptoms. The mean overlap among all instruments was weak (0.21). The overlap between scales ranged from 0.00 to 0.63. 46% of symptoms appeared in only one scale and none of the symptoms were measured by all scales. Scales differed in rate of idiosyncratic symptoms (7-50%), specific symptoms (25-100%) and core GAD symptoms (0-40%).

Of the seven self-report scales included only two were developed specifically to measure GAD: GAD-7 and GAD-D. Among the seven scales that are widely used in GAD research there is a high heterogeneity and low overlap. This may pose a threat to the generalizability of research results due to the low content overlap.