Included articles were grouped in the following areas of relevance : ( @ ) biological validity of illness , ( @ ) baseline severity of illness , ( @ ) chronicity of the index episode of depression , ( @ ) age of participants , ( @ ) medical and psychiatric comorbidity , ( @ ) probability of receiving placebo , ( @ ) use of prospective treatment phases ( lead-in ) ( @ ) dosing schedule , ( @ ) trial duration , ( @ ) frequency of follow-up assessments , and ( @ ) study outcome measure .