Systematic reviews of evaluations of prognostic variables

This is the last in a series of four articles Prognostic studies include clinical studies of variables predictive of future events as well as epidemiological studies of aetiological risk factors. As multiple similar studies accumulate it becomes increasingly important to identify and evaluate all of the relevant studies to develop a more reliable overall assessment. For prognostic studies this is not straightforward. Box 1 summarises the clinical importance of information on prognostic factors. Many of the issues discussed are also relevant to aetiological studies, especially cohort ones. Some features of prognostic studies lead to particular difficulties for the systematic reviewer. Firstly, in most clinical prognostic studies the outcome of primary interest is the time to an event, often death. Meta-analysis of such studies is rather more difficult than that for binary data or continuous measurements. Secondly, in many contexts the prognostic variable of interest is often one of several prognostic variables. When examining a variable of interest researchers should consider other prognostic variables with which it might be correlated. Thirdly, many prognostic factors are continuous variables, for which researchers use a wide variety of methods of analysis. #### Summary points Systematic reviews are applicable to all types of research design, and studies of prognostic variables are an important additional area where appropriate methodology should be applied Prognostic variables should be evaluated in a representative sample of patients assembled at a common point in the course of their disease—ideally they should all have received the same medical treatment or been in a randomised trial When examined critically, a high proportion of prognostic studies are found to be methodologically poor Meta-analysis of published data is hampered by difficulties in extraction of data and variation in the characteristics of the study and patients The poor quality of the published literature is a strong argument in favour …