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The aims of this paper are twofold. Firstly, to describe the obstacles that lie in the way of the researcher establishing the truth and how these may be dealt with in terms of research design. Secondly, to provide an overview of the different designs and the trade offs involved.
The aim of research should be to establish the truth and research design aims to minimise or exclude the threats to the internal validity of the study (that is, that the conclusions are warranted by the observations). These threats are bias, confounding, and chance.
BIAS
Bias is a systematic deviation from the truth that distorts the results of research. 1 Bias can occur anywhere within the research process and here are some common sources.
Selection bias
This occurs when the study subjects differ systematically from the population with the same condition. For example, subjects who present to hospital may not be representative of all patients with the condition and this affects the ability to generalise the result outside the study sample (that is, external validity). Similarly, those who volunteer for studies are different from those who refuse usually being healthier and better educated than the population as a whole.
Intervention bias
For example, the greater use of diagnostic or treatment procedures on the favoured arm in a trial may overestimate the benefit of the intervention. Conversely those patients who are poorly compliant with their intervention reduce the chances of it being effective.
Follow up bias
Those patients who remain in a study may differ from those lost both in terms of personal characteristics and outcome status. Those lost may have died or not wished to be followed up because of the treatment.
Measurement and information bias
This entails misclassifying according to disease or exposure or both. Thus if an investigator knows of the exposures or treatments received this could influence his assessment of the outcome. Similarly, knowledge of the outcome may influence his assessment of exposure.
Some 35 types of bias have been described and the interested reader should consult the paper by Sackett. 2 The key to decreasing bias is to identify the possible areas that could be affected and to change the design accordingly. Increasing the sample size will not reduce bias. Bias...