A trial is to be set up to examine a new drug for the maintenance of remission in all people with ulcerative colitis. A randomised double blinded placebo controlled trial recruits patients in hospital with an exacerbation of their ulcerative colitis. A standard questionnaire is used to record the frequency of exacerbations of disease over a two year period. Results show that there is no difference between the drug and placebo and the results are published in a medical journal. To which form of bias is this study most susceptible? Allocation bias Assessment bias Observer bias Recall bias Selection bias correct answer Bias occurs when there is a systematic difference between the results from a randomised controlled trial and the true state of affairs. In this case the study population (people in hospital with a flare of UC) is different from the population to whom the results will be applied (all people who have UC) and there is therefore said to be a selection bias. Allocation bias occurs when patients are not randomly assigned to a particular treatment. Assessment bias occurs when the observer knows which treatment the subject is taking. Observer bias is when one observer consistently under or over reports a particular variable. Recall bias applies to case-control studies when a patient is more likely to remember a particular detail of exposure if they go on to develop the disease. It has been argued that the early results of trials of HRT demonstrating potentially positive results on reduction in cardiovascular risk are related to selection bias as women of higher social classes who exercised more were likely to choose HRT.
Posted on: Wed, 26 Feb 2014 05:29:28 +0000
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