Types of Research Study

The bulk of the definitions on this page are taken from Green S, Higgins J, editors. Glossary. (2005). Cochrane Handbook for Systematic Reviews of Interventions 4.2.5 [updated May 2005]. 

Anecdotal Report

Subjective, usually unpublished, observations. Generally considered to be less reliable than objective, scientific studies.


'A study that compares people with a specific disease or outcome of interest (cases) to people from the same population without that disease or outcome (controls), and which seeks to find associations between the outcome and prior exposure to particular risk factors. This design is particularly useful where the outcome is rare and past exposure can be reliably measured. Case-control studies are usually retrospective, but not always.'

Considered to be good quality because risk of bias is less than most other forms of study, although still higher than random control trial.

Case Report

Also known as Anecdote, Case History, or Single Case Report.

'A study reporting observations on a single individual.'

Considered to be low quality because risk of bias is higher than other forms of study.

Case Series

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

Considered to be low quality because risk of bias is higher than other forms of study.

Clinical Trial

Also known as an intervention study.

Clinical trials are pre-planned studies of the safety, efficacy, or optimum dosage schedule (if appropriate) of one or more diagnostic, therapeutic, or prophylactic drugs, devices, or techniques selected according to predetermined criteria of eligibility and observed for predefined evidence of favourable and unfavourable effects.

Clinical trials may include a variety of designs such as uncontrolled trials, controlled trials, and randomised controlled trials.

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.'

Considered to be good quality because risk of bias is less than most other forms of study, although still higher than random control trial.

Comparative Study

Any kind of study which involves a comparison of outcomes, results, responses, etc for different techniques, therapeutic approaches or other inputs.

Controlled Trial

'A clinical trial that has a control group. Such trials are not necessarily randomised.'

Cross-Over Trial

'A type of clinical trial comparing two or more interventions in which the participants, upon completion of the course of one treatment, are switched to another.'

Experimental Study

'A study in which the investigators actively intervene to test a hypothesis.'


'The use of statistical techniques in a systematic review to integrate the results of included studies. Sometimes misused as a synonym for systematic reviews, where the review includes a meta-analysis.'

Multicentre Trial

A trial conducted at several geographical sites. Trials are sometimes conducted among several collaborating institutions, rather than at a single institution - particularly when very large numbers of participants are needed.

Multiple Baseline Study

A multiple-baseline design is essentially a series of A-B designs that are replicated in one of three ways: (a) with the same individual across different behaviors; (b) with the same individual across different settings; and (c) with the same behavior across different individuals. A multiple-baseline design strengthens the hypothesis that an intervention caused a behavior change but does not allow for statements of causality

Source: Polaha, J. A., and Allen, K. D. (1999). A tutorial for understanding and evaluating single subject methodology. Proven Practice: Prevention and Remediation Solutions for Schools, 1(2), pp. 73-77.

Non-Randomised Study

'Any quantitative study estimating the effectiveness of an intervention (harm or benefit) that does not use randomisation to allocate units to comparison groups (including studies where 'allocation' occurs in the course of usual treatment decisions or peoples' choices, i.e. studies usually called 'observational').'

Observational Study

'A study in which the investigators do not seek to intervene, and simply observe the course of events. Changes or differences in one characteristic (e.g. whether or not people received the intervention of interest) are studied in relation to changes or differences in other characteristic(s) (e.g. whether or not they died), without action by the investigator. There is a greater risk of selection bias than in experimental studies.'

Open Clinical Trial

Also known as Open Trial or Open Label Trial

'There are at least three possible meanings for this term:

'1. A clinical trial in which the investigator and participant are aware which intervention is being used for which participant (i.e. not blinded). Random allocation may or may not be used in such trials. Sometimes called an 'open label' design.

'2. A clinical trial in which the investigator decides which intervention is to be used (non-random allocation). This is sometimes called an open label design (but some trials which are said to be 'open label', are randomised).

'3. A clinical trial that uses an open sequential design.'

Primary Study

Also known as Original Study.

' "Original research" in which data are collected. The term primary study is sometimes used to distinguish it from a secondary study (re-analysis of previously collected data), meta-analysis, and other ways of combining studies (such as economic analysis and decision analysis).

Prospective Study

'In evaluations of the effects of healthcare interventions, a study in which people are identified according to current risk status or exposure, and followed forwards through time to observe outcome. Randomised controlled trials are always prospective studies. Cohort studies are commonly either prospective or retrospective, whereas case-control studies are usually retrospective.'

Random Control Trial

Also known as Randomised Control Trial or RCT

'An experiment in which two or more interventions, possibly including a control intervention or no intervention, are compared by being randomly allocated to participants. In most trials one intervention is assigned to each individual but sometimes assignment is to defined groups of individuals (for example, in a household) or interventions are assigned within individuals (for example, in different orders or to different parts of the body '

Considered to be high quality because risk of bias minimised.

Retrospective Study

'A study in which the outcomes have occurred to the participants before the study commenced. Case-control studies are usually retrospective, cohort studies sometimes are, randomised controlled trials never are. See also prospective study.'


Also known as overview

'A review article in the medical literature which summarises a number of different studies and may draw conclusions about a particular intervention. Review articles are often not systematic.'

Secondary Study

'A study of studies: a review of individual studies (each of which is called a primary study). A systematic review is a secondary study.'

Systematic Review

Also known as a systematic overview

'A review of a clearly formulated question that uses systematic and explicit methods to identify, select, and critically appraise relevant research, and to collect and analyse data from the studies that are included in the review. Statistical methods (meta-analysis) may or may not be used to analyse and summarise the results of the included studies.'

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03 May 2022