Table 1.

Tips to avoid different types of bias during a trial.

Type of Bias	How to Avoid
Pre-trial bias
Flawed study design	• Clearly define risk and outcome, preferably with objective or validated methods. Standardize and blind data collection.
Selection bias	• Select patients using rigorous criteria to avoid confounding results. Patients should originate from the same general population. Well designed, prospective studies help to avoid selection bias as outcome is unknown at time of enrollment.
Channeling bias	• Assign patients to study cohorts using rigorous criteria.
Bias during trial
Interviewer bias	• Standardize interviewer's interaction with patient. Blind interviewer to exposure status.
Chronology bias	• Prospective studies can eliminate chronology bias. Avoid using historic controls (confounding by secular trends).
Recall bias	• Use objective data sources whenever possible. When using subjective data sources, corroborate with medical record. Conduct prospective studies because outcome is unknown at time of patient enrollment.
Transfer bias	• Carefully design plan for lost-to-followup patients prior to the study.
Exposure Misclassification	• Clearly define exposure prior to study. Avoid using proxies of exposure.
Outcome Misclassification	• Use objective diagnostic studies or validated measures as primary outcome.
Performance bias	• Consider cluster stratification to minimize variability in surgical technique.
Bias after trial
Citation bias	• Register trial with an accepted clinical trials registry. Check registries for similar unpublished or in-progress trials prior to publication.
Confounding	• Known confounders can be controlled with study design (case control design or randomization) or during data analysis (regression). Unknown confounders can only be controlled with randomization.