Hamilton had a strong personal interest in factor analysis and identified four main factors: a general factor, a second that contrasted agitated and retarded depression, a third that covered insomnia and fatigability, and a fourth that included other symptoms (1, pp58–59; 2, pp288–289; 28). Many subsequent authors have reported factor analyses of the HRSD and review articles have been produced (10; 12; 29). One meta-analysis of 12 studies showed that the number of factors extracted ranges from three to seven, with a modal value of six (10, Table 6). Hedlund and Vieweg described two consistent factors: a general severity of depression factor and a bipolar factor that distinguishes reactive from endogenous depression (12, p157). Several analyses identify factors in the HRSD that correspond to anxiety (28; 30).