Table 1.

Types of Bias Summary.

Bias	Summary	Example
Selection bias	Bias due to the methods used to assign patients to study treatment groups.	A surgeon in a glaucoma laser versus topical medicine RCT can accurately guess the allocation of future patients. They may then preferentially wait to identify the “ideal” patient for each treatment arm, opposed to having them assigned at random.
Performance bias	Bias that occurs when patients or clinicians are aware of the assigned treatment, and perform differently as a result.	A patient learns that they received the placebo treatment in a study. When they are performing a visual acuity test they, consciously or subconsciously, do not perform their best due to knowing they received a null treatment.
Detection bias	Bias in the measurement of study outcomes when outcome assessors are aware of the assigned treatment.	A surgeon grading post operative inflammation in an ophthalmology RCT is not masked to the patient’s treatment, and this knowledge influences their assessments based on prior knowledge and experiences.
Attrition bias	Bias due to an influencing factor that causes non-random withdrawals from the study groups.	A study assessing visual acuity after retinal detachment has a large number of withdrawals that occurred primarily in patients of lower socioeconomic status.
Reporting bias	Bias in the outcomes reported by a study, mainly when non-significant findings are ignored.	A published RCT on cataract surgery stated that they would assess visual acuity, adverse events, and quality of life within their protocol; however, only visual acuity and adverse event outcomes are reported in the manuscript.