Bias in clinical trials

Answers

Answers a, b, c, d, and e are all true.

Allocation bias is the systematic difference between participants in how they are allocated to treatment. Allocation bias did not occur as participants were randomised to the intervention (answer a is true). Each individual, therefore, had the same probability of being allocated wedged insoles or control insoles. As a result of random allocation, systematic differences in confounding factors between treatment groups at baseline were minimised, although not necessarily eliminated. Hence any differences in outcome between treatment groups when the trial ended would have been due to differences in treatment and not to differences in characteristics at baseline. Allocation bias would have occurred, for example, if the researchers allocated those individuals to wedged insoles whom they thought would show the greatest benefit with that intervention. The researchers might have done this, for example, because they favoured wedged insoles and wished to show that they were more effective than flat insoles.

Ascertainment bias is the systematic distortion of the assessment of outcome measures by the investigators or trial participants because they are aware of treatment allocation. Ascertainment bias is sometimes referred to as detection bias. Because the trial was double blind, and it was unlikely that the investigators or participants became aware of treatment allocation, then ascertainment or detection bias would have been minimised (b and c are true).

Ascertainment bias would have occurred, for example, if the researchers favoured wedged insoles and wished to show that they were more effective. If the investigators were aware of treatment allocation, they could have been biased in their assessment—subconsciously or otherwise—towards the intervention of wedged insoles. Ascertainment bias would also have occurred, for example, if participants knew their treatment allocation; they might have been disappointed if allocated the control treatment of flat insoles and in their subjective report of pain might have given worse scores than were actually experienced, resulting in an exaggerated difference in outcome between the treatments. When ascertainment bias occurs on behalf of the investigators it is called assessor bias, and when on behalf of the participants it is called response bias. Therefore, because the trial was double blind, both assessor and response bias would have been minimised (d and e are true).