Measurement of Information Bias

4b Measurement or Information Bias. Measurement bias occurs when the classification of exposure or disease is not valid, which can occur because of errors by the observer, the respondent, or the instrument used to measure exposure or disease (e.g., questionnaire, laboratory assay). Misclassifications can be nondifferential or differential. Nondifferential misclassification occurs when the exposure or disease is classified incorrectly in the same proportions in the two comparison groups—for example, in both cases and controls. This type of error will usually bias the RR toward the null hypothesis and make it harder to detect a true effect and can also hinder the ability to observe exposure response relationships. Nondifferential misclassification bias can occur in both cohort and case-control studies. The use of pathological or histological reports to classify disease can reduce disease misclassification, whereas the use of biomarkers to measure exposure or outcome can reduce exposure misclassification (see Section 26.3 for a discussion of biomarkers). 

Differential misclassification occurs when the classification of disease is dependent on the exposure status or the classification of exposure is dependent on the disease status. Differential misclassification can bias the RR in either direction, and often the direction is unknown. Some examples of differential misclassification of exposure are recall bias and observer bias. Recall bias, which is limited to case-control studies, occurs when the cases remember exposure differently than healthy controls this type of bias usually results in finding a greater effect than what is real. Observer bias can occur if the observers, such as study interviewers, incorrectly assign exposure because they know the outcome status of an individual, or it can occur in the follow-up of disease if the observer knows the exposure status of the subject. Ideally, the observer should be blind to the outcome or exposure status of the study subjects. 

The third major category of bias, Publication bias, applies to the evaluation of a putative risk factors across studies that are reported in the published literature. Positive findings are probably published more often than negative studies, and thus a meta analysis or review of the literature would be biased toward finding a greater effect than the true effect. 

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