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I2 Statistic: A statistical test used to quantify heterogeneity in a meta-analysis.39 It describes the percentage of variability in effect estimates due to heterogeneity rather than sampling error (chance).6 Also called I2 index. See also Cochran’s Q test.

Incidence: The number of new cases of an event that develop within a given time period in a defined population at risk, expressed as a proportion.1

Intention-to-treat (ITT) analysis: In a randomized controlled trial, the statistical analysis of all participants based on the group to which they were originally assigned.4, 6 This minimizes bias caused by the loss of participants (attrition) that may disrupt the baseline equivalence established by randomization. The term is often misused in trial publications when some participants were excluded.

Intermediate outcome: See surrogate endpoint.

Internal consistency: See reliability.

Internal validity: The extent that the design and conduct of a study are likely to have prevented bias.6 More rigorously designed (better quality) trials are more likely to yield results that are closer to the truth. See also validity, bias prevention.

Intervention: A generic term used to describe a program, policy, measure or activity designed to have an impact on an illness or disease in an individual or a population.1 In clinical trials, the term may be used to describe regimens in all comparison groups.6