The power of test

Multiple linear regression: The control of confounding factors

The inclusion of exposure variables that are strongly associated with the outcome variable will reduce the residual variation and hence decrease the standard error of the regression coefficients for other exposure variables. This means that it will increase both the accuracy of the estimation of the other regression coefficients, and the likelihood that the related hypothesis tests will detect any real effects that exist. This attribute is called the power of the test.


Medical statistics, Kirkwood B., and Sterne J., Blackwell publishing, 2006



Systematic reviews

Systematic reviews of research are always preferred; For reliable evidence on rare harms, therefore, we need a systematic review of case reports rather than a haphazard selection of them. Qualitative studies can also be incorporated in reviews
Different types of question require different types of evidence: Randomised trials can give good estimates of treatment effects but poor estimates of overall prognosis; comprehensive non-randomised inception cohort studies with prolonged follow up, however, might provide the reverse.

Hierarchies can lead to anomalous rankings. For example, a statement about one intervention may be graded level 1 on the basis of a systematic review of a few, small, poor quality randomised trials, whereas a statement about an alternative intervention may be graded level 2 on the basis of one large, well conducted, multi-centre, randomised trial. This ranking problem arises because of the objective of collapsing the multiple dimensions of quality (design, conduct, size, relevance, etc) into a single grade.

Whatever evidence is found, this should be clearly described rather than simply assigned to a level. Such considerations have led the authors of the BMJ’s Clinical Evidence to use a hierarchy for finding evidence but to forgo grading evidence into levels. Instead, they make explicit the type of evidence on which their conclusions are based.

To overcome flaws in evidence hierarchies we need to do firstly, to extend, improve, and standardise current evidence hierarchies22; and, secondly, to abolish the notion of evidence hierarchies and levels of evidence, and concentrate instead on teaching practitioners general principles of research that they can use these principles to appraise the quality and relevance of particular studies

Different types of research are needed to answer different types of clinical questions

Irrespective of the type of research, systematic reviews are necessary

Adequate grading of quality of evidence goes beyond the categorisation of research design

Risk-benefit assessments should draw on a variety of types of research

Clinicians need efficient search strategies for identifying reliable clinical research

Source: Assessing the quality of research; BMJ 2004;328;39-41
searched in www.ouls.ox.ac.uk