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Box 13.1.a: Some types of NRS design used for evaluating the effects of interventions

Designs are distinguished below by labels in common use and descriptions are intentionally non-specific because the labels are interpreted in different ways with respect to details. The NRSMG does not advocate using these labels for reasons explained in Section 13.5.1.

Non-randomized controlled trial.

An experimental study in which people are allocated to different interventions using methods that are not random.

Controlled before-and-after study.

A study in which observations are made before and after the implementation of an intervention, both in a group that receives the intervention and in a control group that does not.

Interrupted-time-series study.


A study that uses observations at multiple time points before and after an intervention (the ‘interruption’). The design attempts to detect whether the intervention has had an effect significantly greater than any underlying trend over time.

Historically controlled study.

A study that compares a group of participants receiving an intervention with a similar group from the past who did not.

Cohort study.

A study in which a defined group of people (the cohort) is followed over time, to examine associations between different interventions received and subsequent outcomes. A ‘prospective’ cohort study recruits participants before any intervention and follows them into the future. A ‘retrospective’ cohort study identifies subjects from past records describing the interventions received and follows them from the time of those records.

Case-control study.

A study that compares people with a specific outcome of interest (‘cases’) with people from the same source population but without that outcome (‘controls’), to examine the association between the outcome and prior exposure (e.g. having an intervention). This design is particularly useful when the outcome is rare.

Cross-sectional study.

A study that collects information on interventions (past or present) and current health outcomes, i.e. restricted to health states, for a group of people at a particular point in time, to examine associations between the outcomes and exposure to interventions.

Case series (uncontrolled longitudinal study).

Observations are made on a series of individuals, usually all receiving the same intervention, before and after an intervention but with no control group.