The continuity of depression symptoms: Use of cluster analysis for profile identification in patient and student samples

Background: Findings from several sources suggest that depression lies on a continuum whereby mild and severe variants are thought to differ in severity (i.e., quantitatively), but not in kind (i.e., qualitatively). The current study used cluster analysis to extend this work to examination of depression symptom profiles obtained in distressed student ‘analogue’ samples and clinically depressed samples. Method: Patients with major depressive disorder (n=101) provided seed points for the depressed cluster, and 176 non-distressed university students (Beck Depression Inventory score ≤8) provided seed points for the non-depressed cluster. The symptom profiles of three levels of analogue depressed samples were then evaluated (BDI≥9, BDI≥16, and BDI≥21). Results: Only 35.4% of BDI≥9 analogue respondents were empirically sorted to the depression cluster and the majority were assigned to the non-depressed cluster. The proportion assigned to the depression cluster increased to 70.5% and to 86.2% when higher BDI cutoffs of 16 and 21 were examined, respectively. The DSM-IV depression symptom profile of the BDI≥21 group was very similar to the profile defined by clinical patients. Limitations: The study relied solely on self-report to assess symptom severity. Conclusions: It is recommended that higher BDI cutoffs be utilized in analogue depression research than is currently common. On quantitative grounds, analogue subjects who were sorted to the clinically defined depression cluster seem to best represent the idea of depression continuity.