Research designs


A method for statistically combining the results of similar studies which are included in a systematic review, to come to a conclusion about the overall effects of an intervention. 

Systematic reviews

A comprehensive review of all relevant research about the efficacy of a treatment or intervention; involves systematic and transparent identification, selection, synthesis and appraisal of studies. A systematic review usually involves the synthesis of results from multiple studies, which can be done using meta-analysis.

Randomised controlled trial (RCT)

An experiment in which two or more conditions are compared by randomly allocating participants to one of at least two groups (usually, one group receives an intervention or treatment, and the other – the ‘control group’ – receives no intervention or an alternative intervention), and then testing the effects. RCTs can have more than two groups (e.g. two treatment groups and one control group). RCTs are considered one of the most reliable research designs, because random allocation helps to reduce the risk that the effects of the intervention might have been due to some form of bias. Also see Randomise Me

Cohort study

An observational study in which a defined group of people (the cohort) is followed over time. The outcomes of people in subsets of this cohort are compared, to examine people who were exposed or not exposed (or exposed at different levels) to a particular intervention or other factor of interest. A prospective cohort study assembles participants and follows them into the future. A retrospective (or historical) cohort study identifies subjects from past records and follows them from the time of those records to the present. 

Case-control study

A study that compares people with a specific outcome of interest (cases) to people from the same population without that outcome (controls), and which seeks to find associations between the outcome and prior exposure to particular risk factors. Case-control studies are usually retrospective, which means that they ask participants to recount something they did, rather than delivering a programme or treatment to participants and then measuring what, if any, differences may have occurred as a result (the latter is more common in cohort studies).

Case series and case report

A study reporting observations on a series of individuals, usually all receiving the same intervention, with no control group.

Quantitative research

Research which measures and analyses observations in a numerical way (as opposed to qualitative research, which measures conditions in a non-numerical way).

Qualitative research

Research which is carried out in the field (natural settings) and analysed largely in non-numerical ways. Most qualitative studies aim to generate an in-depth understanding of human behaviour and the reasons that govern such behaviour, investigating people’s motivations, feelings and reactions.

Non-randomised controlled trial

Any quantitative study estimating the effectiveness of an intervention (harm or benefit) that does not use randomisation to allocate participants to comparison groups. To avoid ambiguity, the term should be substantiated using a description of the type of question being addressed. For example, a ‘non-randomised intervention study’ is typically a comparative study of an experimental intervention against some control intervention (or no intervention) that is not a randomised controlled trial. There are many possible types of non-randomised intervention study, including cohort studies, case-control studies, controlled before-and-after studies, interrupted-time-series studies and controlled trials that do not use appropriate randomisation strategies (sometimes called quasi-randomised studies).

Stepped Wedge Randomized Controlled Trials

There is often a tension between policy makers wishing to implement a novel intervention and researchers wanting time to do a controlled evaluation. One trial design that can address this tension is the stepped wedged randomized trial. In a stepped wedge trial, all groups or clusters of participants eventually receive the intervention, but the implementation is staggered in a random fashion allowing a robust evaluation to take place. Recently there has been an upsurge in interest in the design, which is not yet well understood. A series of papers on the Trials Journal website reviews previous stepped wedge studies and explores key issues of design, forming a comprehensive overview of the current state of knowledge of the design.