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Bias Inspector

Potential Biases in a Cohort Study

Testing for Biases in a Cohort Study

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Study Design Phase

Study participants may have responded differently due to the fact that they were observed Subjects who remain in the study differ from those lost to follow-up Available information was used; however, it may not be the most representative of the problem at hand Study participants allocated earlier in time to an intervention or a group may be subject to different exposures (or have a different risk) compared to participants who were recruited later The indication for the exposure may be the actual cause of the outcome Higher or lower access to testing may have led to over-estimate or under-estimate the prevalence of disease in some groups A specific subgroup of the population was not available in a register used to identify the source population The topic of the study is considered new and "hot"; therefore, it may be published with less critical approach from investigators and editors Defining a starting time for the exposure/outcome is complicated Inferences were based on a non-representative selection of all the ones that a set of data could produce Key variables in the study may be inaccurately measured or classified When analyzing health records, the exposure or the outcome may have increased the likelihood of having a health record When the literature review was done, some studies were omitted because they were written in a different language The results of a previous assessment, test result, or diagnosis, if known, may have affected the results of subsequent processes on the same patient The exposure/treatment may have been initiated or stopped in response to a symptom of the outcome/disease The variable of interest was replaced by a proxy variable (e.g., using IQ as a proxy measure of intelligence) that might also be influenced by other factors A non-random sampling technique was used, and the likelihood of unusual outcomes may have increased as a result of referrals to the study Systematic differences may exist between the characteristics of the selected participants and those who were not The sensitivity and specificity of the test used may be affected by differences in the characteristics of patients in different settings (e.g., primary care, emergency care, hospital setting) because each of these settings has a different mix of patients The study tends to support the interests of some financial sponsors Participants who volunteered to participate in studies may not represent the general population The wrong sample size was used in the study
Assignment and Data Collection Phase

Some members of the target population were less likely to be included in the final results than others due to data collection methods Knowledge of a subject’s prior exposures or personal biases may have influenced both the process and the outcome of diagnostic tests The method, tool, or test used to detect the outcome lacks accuracy to discern a clinically significant change or difference The exposure may cause a benign disorder that resembles the disease The probability of misclassified participants may be different across study groups Systematic differences may exist between responders and non-responders Information observed and recorded may include systematic discrepancies from reality Data collection and interpretation might be subjective Study groups may have received unequal care that might affect the outcome or may have behaved differently just because of the assigned group There might be differences in the uptake of healthcare as a result of public interest in a disease or condition and its possible causes Participants may have felt embarrassed or ashamed to report the exposure or disease The exposure caused symptoms resulting in the search for the outcome The results of a diagnostic test may be affected whether or not the gold standard procedure is used to verify the test result
Data Analysis and Reporting Phase

The study may have a controversial result that was omitted or played down A variable that can be caused by both the exposure and the outcome (i.e., collider) was controlled for in the analysis Personal beliefs or hypotheses may have guided the literature review, interpretation of results, or information recall An extraneous factor that can be associated with both the exposure and the outcome (confounder) is not controlled for in the analysis There were systematic differences between groups in how outcomes were determined When evaluating a screening test, survival time was compared between screened and unscreened patients regardless of the lead time When evaluating a screening test, screening may have identified less aggressive cases, which makes them seem to have lived longer than non-screened cases References may be restricted only to research that supports the authors' position The authors tend to submit and publish positive results rather than non-significant or negative results The research paper is subject to selective disclosure or withholding of information by parties involved with regards to the topic of the study, its design, conduct, or analysis Researchers may have overinterpreted or misinterpreted their results to serve their research hypothesis

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