Table 2.

Sources of error in pharmacoepidemiology studies

Source of Error	Definition	Question To Be Answered
Confounding	An observed association, or lack of association, that is due to a mixing of effects between the exposure, the outcome, and a third confounding variable	Is there a third factor associated with both the treatment (exposure) and the outcome?
Confounding by indication	Systematic error that occurs when the disease itself, or symptoms of the disease, are risk factors for the outcome being studied.	Is the underlying disease being treated in the study a risk factor for the outcome?
Selection bias	Systematic error that arises from methods to select participants for a study that is related to the probability of developing the outcome of interest.	Were the two study groups selected into the study similar, with the exception of the exposure of interest?
Information bias	Systematic error that is associated with the measurement of the exposure or outcome.	Were data on the exposure and outcome measured/collected the same way in both groups?
Generalizability (external validity)	The applicability of the results to other populations	Do the results apply to the general population? Your patient population?
Type I (alpha error)	The probability of finding a significant association when the association is actually due to chance	Were the observed results due to chance alone?
Type II (beta error)	The probability of concluding that there is no difference when a real difference exists	What magnitude of effect was the study powered to detect?
Confidence interval	The range within which the true magnitude of the effect exists	Does the confidence interval include/exclude the relative risk that is important to detect?
Precision	The accuracy of the measured results, including the width of the 95% confidence interval	What was the range of results statistically consistent with the observed finding?

Adapted from Gelfand and Langan (2013).