Finding and appraising qualitative evidence
Evidence from Randomised Trials and Systematic Reviews< Back to search results
- Format Texts
- Language/s English
- Target Audience Further education
- EBM Stage 0 - Why EBM?
- Duration <5 mins
- Difficulty Introductory
Key Concepts addressed
- 2-2a Reviews of fair comparisons should be systematic
- 2-1d People should not know which treatment they get
- 1-2f Consider all of the relevant fair comparisons
The main threats to validity in non-randomised studies is related to BIAS due to differences in the populations of patients who do and do not receive the experimental treatment. Randomisation should overcome this problem because the random allocation of patients to the treatment or control groups should create an equal spread of known and unknown risk factors between the two groups. Whilst statistical techniques can be used to adjust for known confounding factors in non-randomised studies, by definition the unknown ones can only be overcome with randomisation.
Even in randomised controlled trials it is important to check that the allocation of patients to the active and comparison groups is well concealed. The quality of allocation concealment is routinely used by the Cochrane Collaboration in grading trials included in systematic reviews because empirical research has shown that studies which do not have well concealed allocation tend to show more inflated results than those that do. Why should this be? The problem is selection bias: if I was carrying out a randomised trial of my favourite wart paint it is important that I do not know which treatment the next patient will receive, otherwise I can influence the results by choosing the milder wart infections for treatment with the paint. This is quite easy in practice as I would only have to find an excuse to rule the next patient out of the trial if they were due to have the paint and had a horrendous crop of warts!
Similarly if I know the treatment used I may be more optimistic in deciding that a wart has completely gone if the patient had my special paint than if they did not. This is a form of detection bias. Secure double blinding (using an identical wart paint substitute prepared by an outside agency) will overcome both problems, and again has been shown to reduce the size of treatment effects compared with the results of unblinded (open) studies.
From Dr Chris Cates, EBM Website.