If you want to improve it, first, you must measure it. In 2002, Centers for Medicare & Medicaid Services (CMS) and the Agency for Healthcare Research and Quality developed the Hospital Consumer Assessment of Healthcare Providers and Systems survey, which is a survey of patient's regarding the quality of their hospital care. As CMS incentivized the collection and reporting of this survey data, these markers of quality care have ultimately become linked to payments to hospitals, and thus have gained a lot of interest. A large body of research is focused on measuring these types of variables that are not directly observable. How should we measure something like patient satisfaction or the quality of care given in hospitals? How do we know that what we are measuring is actually what we are trying to measure and the instrument we are using to measure helps us produce results that are reproducible when given the same scenario? These types of questions are the subjects of validity and reliability studies. There are several different types of reliability studies. In this chapter, we will cover test–retest reliability, internal consistency, and interrater reliability.