Particularly when the results of a clinical research paper are conflicting or counter-intuitive, even an approximate estimate of its strength is valuable.

In general, the rarer the type of study, the greater its strength. The main types of clinical research in descending order of frequency are case reports, case series, case-control studies, cohort studies and meta-analyses[1].

Case reports may nevertheless be invaluable in alerting other physicians to hitherto unrecognised associations, as occurred with limb deformities and maternal Thalidomide.

Case series can similarly give an indication of trends, but supposed causal connections may prove spurious on more critical analysis.

Case-control attempts to remove recognised confounding variables by looking at statistically valid differences between a population of patients and a population of patients or healthy individuals pair-matched except for the feature being studied.

Cohort removes further selection bias by following sufficiently large normal comparable populations over time to track the appearance and progress of disease processes.

Meta-analysis (Medical Litigation News Volume 2, Issue 1) involves the statistical manipulation of data from all available quality studies which contain sufficient detail. By combining the populations studied by each writer, spurious and anomalous findings are eliminated, and trends which are not statistically significant in an individual study may prove to be valid.

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