Question

Discuss bias in analysis and publication of epidemiologic research, and how to reduce such bias.

Your response must be at least 200-400 words in length.

Answer 1

Communication of research findings is the utmost responsibility of all scientists. Publication bias occurs if scientific studies with negative or null results fail to get published. This can happen due to bias in submitting, reviewing, accepting, publishing or aggregating scientific literature that fails to show positive results on a particular topic. Publication bias can make scientific literature unrepresentative of the actual research studies. This can give the reader a false impression about the beneficial effects of a particular treatment or intervention and can influence clinical decision making. Publication bias is more common than it is actually considered to be, but there are ways to detect and prevent it.

Bias in Epidemiological Studies

While the results of an epidemiological study may reflect the true effect of an exposure on the development of the outcome under investigation, it should always be considered that the findings may in fact be due to an alternative explanation^.

Such alternative explanations may be due to the effects of chance, bias or confounding which may produce spurious results, leading us to conclude the existence of a valid statistical association when one does not exist or alternatively the absence of an association when one is truly present^.

Observational studies are particularly susceptible to the effects of chance, bias and confounding and these factors need to be considered at both the design and analysis stage of an epidemiological study so that their effects can be minimised.

Bias

Bias may be defined as any systematic error in an epidemiological study that results in an incorrect estimate of the true effect of an exposure on the outcome of interest.

• Bias results from systematic errors in the research methodology.
• The effect of bias will be an estimate either above or below the true value, depending on the direction of the systematic error.

Common types of bias in epidemiological studies

Information bias

Information bias results from systematic differences in the way data on exposure or outcome are obtained from the various study groups. This may mean that individuals are assigned to the wrong outcome category, leading to an incorrect estimate of the association between exposure and outcome.

Observer bias may be a result of the investigator’s prior knowledge of the hypothesis under investigation or knowledge of an individual's exposure or disease status. Such information may influence the way information is collected, measured or interpretation by the investigator for each of the study groups.

Recall (or response) bias - In a case-control study data on exposure is collected retrospectively. The quality of the data is therefore determined to a large extent on the patient's ability to accurately recall past exposures. Recall bias may occur when the information provided on exposure differs between the cases and controls. For example an individual with the outcome under investigation (case) may report their exposure experience differently than an individual without the outcome (control) under investigation.

Selection bias

Selection bias occurs when there is a systematic difference between either:

• Those who participate in the study and those who do not (affecting generalisability) or
• Those in the treatment arm of a study and those in the control group (affecting comparability between groups).

That is, there are differences in the characteristics between study groups, and those characteristics are related to either the exposure or outcome under investigation. Selection bias can occur for a number of reasons.

Sampling bias describes the scenario in which some individuals within a target population are more likely to be selected for inclusion than others. For example, if participants are asked to volunteer for a study, it is likely that those who volunteer will not be representative of the general population, threatening the generalisability of the study results. Volunteers tend to be more health conscious than the general population.

Epidemiologic research bias can be reduced considering the below points;

All epidemiological studies, even randomised clinical trials, are susceptible to bias (systematic error). The objective of the epidemiologist will be to minimise these biases. This can be done by considering, at the different stages of development and execution of a study, where and how bias may occur: the design stage (protocol writing), subject selection (case/control, exposed/unexposed, intervention/control group etc), data collection, data analysis and interpretation of results.

At the design stage, bias should be considered at the time of protocol writing. A lot of care should be given, at this stage of development of the study, to forecasting all potential selection and information biases that may be encountered. Despite all precautions taken, some biases will persist. They then need to be taken into account in the interpretation of the results of the study.

When writing the report or manuscript, sources of potential bias in the study absolutely need to be openly discussed. Particularly, the first part of the discussion section of a scientific paper should include a detailed paragraph in which authors discuss all potential biases which could have falsely led to the study results. If possible, the direction of the bias (overestimation or underestimation) and the magnitude of the bias also should be discussed.  

While case-control and cohort studies are both susceptible to bias, the case-control study is affected by more sources of bias.