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Incidence
The number of new occurrences of something in a population over a particular period of time, e.g. the number of cases of a disease in a country over one year.

Index Medicus
Catalogue of the United States National Library of Medicine (NLM), and a periodical index to the medical literature. Available in printed form, or electronically as MEDLINE.

Individual patient data
[In meta-analysis:] The availability of raw data for each study participant in each included study, as opposed to aggregate data (summary data for the comparison groups in each study).  Reviews using individual patient data require collaboration of the investigators who conducted the original studies, who must provide the necessary data.

Intention-to-treat
A strategy for analysing data from a randomised controlled trial. All participants are included in the arm to which they were allocated, whether or not they received (or completed) the intervention given to that arm. Intention-to-treat analysis prevents bias caused by the loss of participants, which may disrupt the baseline equivalence established by randomisation and which may reflect non-adherence to the protocol. The term is often misused in trial publications when some participants were excluded.

Interaction
The situation in which the effect of one independent variable on the outcome is affected by the value of a second independent variable. In a trial, a test of interaction examines whether the treatment effect varies across sub-groups of participants. See also factorial trial, sub-group analysis.

Intermediary outcomes
See surrogate endpoints

Internal validity
The extent to which the design and conduct of a study are likely to have prevented bias. Variation in quality can explain variation in the results of studies included in a systematic review. More rigorously designed (better quality) trials are more likely to yield results that are closer to the truth. (Also called methodological quality but better thought of as relating to bias prevention.) See also external validity, validity,  bias prevention.

Inter-rater reliability
The degree of stability exhibited when a measurement is repeated under identical conditions by different raters. Reliability refers to the degree to which the results obtained by a measurement procedure can be replicated. Lack of inter-rater reliability may arise from divergences between observers or instability of the attribute being measured. See also Intra-rater reliability.

Interrupted time series
A research design that collects observations at multiple time points before and after an intervention (interruption). The design attempts to detect whether the intervention has had an effect significantly greater than the underlying trend.

Intervention
The process of intervening on people, groups, entities or objects in an experimental study. In controlled trials, the word is sometimes used to describe the regimens in all comparison groups, including placebo and no-treatment arms. See also treatment, experimental intervention and control.

Intervention group
A group of participants in a study receiving a particular health care intervention.  Parallel group trials include at least two intervention groups.

Intervention study
See Clinical trial.