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Harm: The totality of all possible adverse consequences of an intervention.2

Hazard rate: The probability of an event occurring given that it has not occurred up to the current point in time.6

Hazard ratio: Represents the increased risk with which one group is likely to experience the outcome of interest.6 A measure of effect produced by a survival analysis. For example, if the hazard ratio for death for a treatment is 0.5, then treated patients are likely to die at half the rate of untreated patients.

Head-to-head trial: A controlled trial that compares two active treatments.11

Health outcomes: Encompasses clinical, surrogate, and humanistic outcomes.35 Examples include mortality, physiologic measures, clinical events, symptoms, functional measures, and patients’ experience with care.

Health status: Functional capacity or a state of physiological and psychological functioning or well-being.1

Health technology assessment (HTA): A form of policy research that examines short-and long-term consequences of application of a health care technology.1 The goal of HTA is to provide policymakers with information on new treatments or interventions.

Health-related quality of life (HRQOL): A broad theoretical construct developed to explain and organize measures regarding evaluation of health status, attitudes, values, and perceived levels of satisfaction and general well-being related to either specific health conditions or life as a whole from the individual’s perspective.1 See also patient-reported outcomes.

Heterogeneity: A general term used to describe variation or diversity among studies.6, 36, 37 Heterogeneity should be distinguished as clinical (differences between studies in key characteristics of the participants, interventions, or outcome measures), methodological (differences in study design, conduct, and quality), or statistical (differences in reported effects). Statistical heterogeneity refers to the degree of variation in the effect estimates from a set of studies; it is also used to indicate the presence of variability among studies beyond the amount expected due solely to chance.

Heterogeneous: Describes a set of studies or participants with sizeable heterogeneity.6 The opposite of homogeneous.

Historical control: Previously collected observations used as control values against which treatment values are compared.4, 6 Risk of bias associated with historical controls relates to systematic differences between the comparison groups due to changes over time (e.g., in risks, prognosis, health care, etc.)

Homogeneous: 1. Similarity of participants, interventions, and measurement of outcomes across a set of studies.6 2. In meta-analysis, used specifically to describe the effect estimates from a set of studies where they do not vary more than would be expected by chance.6

Humanistic intermediary: Factors that affect the formation of patients’ opinions about the effects of disease or treatment on their lives and well-being (e.g., values, norms, perceptions).1

Humanistic outcomes: Patient self-assessment of the impact of disease or treatment on their lives and well-being (e.g., satisfaction, quality of life).1 See also patient-reported outcomes (PRO).

Hypothesis: A conjectural statement of the relation between two or more variables.38 A proper hypothesis should be pre-specified, measurable, have theoretical or empirical support, be clearly articulated, and testable by an appropriately designed study.6 See also null hypothesis.