Quality: The extent to which all aspects of a study’s design and conduct can be shown to protect against systematic and nonsystematic bias and inferential error.51 See also bias prevention and internal validity.

Quality of care: The degree to which health services for individuals and populations increase the likelihood of desired health outcomes and are consistent with current professional knowledge.40

Quality score: A value assigned to represent the validity of a study either for a specific criterion or overall.6 Quality scores are expressed as letters (A, B, C) or numbers. See also bias prevention.

Quality-adjusted life year (QALY): A universal health outcome measure applicable to all individuals and all diseases that allows for comparisons across diseases and interventions.1 One QALY is a year of life with no disability (i.e., perfect health). Cumulated across multiple years, the QALY combines, in a single measure, gains or losses in both quantity of life (mortality) and quality of life (morbidity).
Quantitative synthesis: See meta-analysis.

Quasi-experiment: A study that is similar to a true experiment except that it lacks random assignment of participants to treatment and control groups.52, 53 A quasi-experimental design may be used to reveal a causal relationship in situations where the researcher is unable to control all factors that might affect the outcome. Because full experimental control is lacking, the researcher must thoroughly consider threats to validity and uncontrolled variables that may account for the results.