Study design

Description

Randomised/Quasi‐randomised controlled trial

A prospective study where participants are allocated to intervention or control groups and followed up over time to assess any differences in outcome rates (the outcomes of interest are compared between groups at the end of the follow‐up time). In a randomised trial appropriate methods are used for randomisation and concealment of allocation (e.g. use of a telephone or computer system to conceal randomisation, centrally controlled randomisation, pre‐numbered or coded identical containers administered serially to participants, sequentially numbered sealed opaque envelopes). It avoids selection bias because it ensures that on average both known and unknown determinants of outcome (prognostic factors) are randomly distributed between groups. In a quasi‐randomised trial the method of allocation falls short of genuine randomisation and allocation concealment, i.e. the allocation procedure is entirely transparent before assignment (e.g. allocation by date of birth, hospital record number, alternation, odd‐even numbers, patient social security numbers, days of the week, etc).

Non‐randomised trial

A prospective study where the investigator has control over the allocation of participants to groups, but does not attempt randomisation or quasi‐randomisation to allocate participants (e.g. patient or physician preference, patient characteristics and clinical history).

Controlled before‐and‐after study

A non‐randomised prospective study where a control population of similar characteristics and performance as the intervention group is identified. Data are collected in both the control and intervention groups before (at baseline) and after the intervention, and either final values (if the groups are comparable at baseline) or change from baseline values are compared. The before and after measurements may be made in the same participants or in different samples. An example of the before‐after design: a study in which 2 groups of 12‐year olds from 2 similar populations were examined for prevalence of caries prior to initiating salt fluoridation in 1 of the groups, and 5 years after starting fluoridation 12 year olds are examined in the 2 areas (1 fluoridated, the other not) and caries in the 2 groups/areas are compared. It is important to note that here the children are different in the before and after periods.

(Concurrent) Cohort study

A follow‐up study that compares outcomes of participants in subsets of the cohort ‐ to examine people who were exposed or not exposed (or exposed at different levels) to a particular intervention (or other factor of interest). Participants are studied during the same (concurrent) period prospectively or retrospectively. A prospective cohort study assembles participants and follows them into the future. A retrospective cohort study identifies subjects from past records and follows them from the time of those records to the present. An example of a prospective cohort study: 2 groups of children (subsets of a cohort) are examined at age 12 prior to the initiation of fluoridation of salt for 1 group, and these same children are followed up with annual examinations for caries for 4 years.

Study design

Description

Historical control study

A variation on the traditional cohort study where the outcome from a new intervention/exposure is established for participants studied in 1 period and compared to those who did not received the intervention in a previous period, i.e. participants are not studied concurrently as 1 of the groups is taken from the past.

Case‐control study

Participants with and without a given outcome are identified (cases and controls respectively) and exposure to a given factor(s)/intervention(s) are compared between the 2 groups (it is investigated how exposure to suspect agents differed between the 2 groups).

Before‐and‐after study (single group pre/post‐design)

In this type of study there is a comparison of outcomes from study participants before and after they have received an intervention. The before and after measurements may be made in the same participants or in different samples. There is no control group for comparison.

Interrupted time series

Interrupted time series (ITS) studies collect 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. The intervention effect is measured against the pre‐intervention trend. There is no way to assess the impact of any concurrent events on the outcomes of interest.

Cross‐sectional study

A study examining the prevalence of a condition/exposure and/or the relationship between a condition and other variables of interest/other characteristics as they exist in a defined population at a particular point in time.