Infection control studies often rely on infection endpoints to determine whether interventions are effective. However, many infection outcomes, including those defined by standardized surveillance criteria, involve some subjective judgment for determination. Studies that use unblinded ascertainment of subjective infection endpoints are at risk of assessment bias. Unfortunately, infection control studies have not routinely accounted for assessment bias. To ensure validity, infection control studies should incorporate study design elements to control assessment bias, such as blinded assessment or use of objective outcome measures.