Table 1.

Summary of five classic types of bias that are evaluated in primary studies

Type of Bias	Definition	Example	How to Minimize
Reporting bias (publication bias)	There is deviation, typically toward favorable results, in published studies compared with unpublished studies.	Several RCTs evaluate if a new laser delivery improves facial aging compared with an existing treatment. Only the trials with statistically significant improvement are published; studies that encountered complications might also be suppressed.	Inclusion of unpublished data in the systematic review and meta- analysis provides a more balanced review.
Selection bias	Each group is selected differently, causing incomparable groups with regard to important baseline characteristics and predictors of outcome.	An RCT compares the effect of perioperative steroid vs placebo on decreasing facial edema after septorhinoplasty. Patients with more extensive osteotomies and bone mobilization were allocated to the treatment group.	In observational studies, rigorous cohort enrollment and adjustment methods, if required. In RCTs, allocation concealment can prevent biased selection.
Performance bias	Substantial differences in care among groups influence the outcome.	A cohort study evaluates if antibiotic use after laser resurfacing decreases the risk of infection. The treating physicians were not masked and were more likely to add topical antibiotic treatment to the control group.	Masking the health care providers to be unaware of patient allocation and treatment.
Attrition bias	The rate of withdrawal is unequal between the study groups, leading to incomplete data that influence the analysis.	A study compares repeated filler injection with placebo. Patients receiving placebo injections were more likely to drop out or not comply with follow-up.	Masking of patients may decrease aspects related to patient perception.
Detection bias	A systematic difference exists between groups in how the outcome is determined or assessed.	A study compares two methods of rhinoplasty using surgeon-based outcome. Surgeons’ perception and beliefs influence the evaluation of outcome.	Masking the assessors of outcome to minimize the influence of their beliefs and perceptions.