With baseline balance, post-baseline groups differences on illness severity can more safely be attributed to the intervention. The second stage of implementing the propensity adjustment involves treatment effectiveness analyses. As implemented in the examples below, the observations are stratified into quintiles of the propensity score. Unlike unadjusted analyses, stratification involves separate Inhibitors,research,lifescience,medical analyses for each propensity quintile. Effectiveness analyses might be conducted with a t-test of severity ratings or chi-square test of response rates for cross-sectional data. For longitudinal data,

in contrast, mixedeffects linear regression, mixed-effects logistic regression, or mixed-effects grouped time survival models, could be used. The choice among these analytic approaches depends on the form of the dependent variable. In each case, treatment is the primary independent variable. Hie quintile-specific results can be pooled using the Mantel-Haenszel procedure to NVP-AEW541 provide one unified estimate of the treatment Inhibitors,research,lifescience,medical effect. Inhibitors,research,lifescience,medical However, pooling can only be used if the assumption

of no treatment by quintile interaction has been evaluated and supported empirically. As stated earlier matching, inverse probability weighting, and covariate adjustment provide alternatives to stratification. These alternatives are particularly useful if the sample size precludes quintile stratification, which, of course, involves only 20% of the observations in each quintile-specific analysis. Observational studies of antidepressant effectiveness Two examples of observational evaluation of antidepressants are presented below. Each includes two Inhibitors,research,lifescience,medical stages of analyses: a propensity model and a treatment effectiveness model. The former examines the magnitude

and direction of variables hypothesized to be associated with receiving various ordered categorical antidepressant doses. Hie latter examines the antidepressant effect relative to a comparator, no antidepressant in these examples. Each example comes from the National Institute Inhibitors,research,lifescience,medical of Mental Health Collaborative Depression Study (CDS). Hie CDS is a longitudinal, found observational study that recruited 955 subjects from 1978 through 1981 who sought treatment for one of the major mood disorders (major depressive disorder, mania, or schizoaffective disorder) from one of five academic medical centers in the United States (Boston, Massachusetts; Chicago, Illinois; Iowa City, Iowa; New York, New York; and St Louis, Missouri). All subjects were English-speaking, Caucasian, and at least 17 years of age. Each subject provided informed written consent.16 Each example below included up to 20 years of follow-up data. These data capture the repeated antidepressant exposure a patient receives during the chronic course of depression: episodes, recovery periods, and recurrences.