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Tips to avoid different types of bias during a trial.

Type of BiasHow 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 Check registries for similar unpublished or in-progress trials prior to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.