An experiment is a study in which an intervention is deliberately introduced to observe its effects.

Shadish, Cook and Campbell 2002: 12

The researcher observes whether any change that occurs during the experiment is attributable to the intervention or treatment. Although natural experiments do occur - where by accident people or areas get a different intervention - the more common method is where a researcher or agency randomly allocates people or units to control and treatment groups, in what are often called randomised controlled trials (RCTs).

Randomisation ensures that there are no differences in observed outcomes between those in the treatment or control groups other than from the treatment so the researcher may observe an unbiased estimate of the intervention (see figure below). Other experimental research designs include quasi-experiments (which lack random assignment) and design experiments (qualitative research in which an intervention in adapted over time in response to observed outcomes).

Experiments have a long history in the social sciences. Recently, economists, social policy experts, political scientists and public agencies of all kinds have become more interested in experiments because of their ability – in theory at least – to offer clear and reliable evaluations of human interventions.

Groups exploring experiments


Seminars were organised to start building a network of researchers undertaking field or laboratory experiments in Manchester, with a focus on social research with human participants (funded by methods@manchester).

  • Experiments Research Network Launch Seminar (19 January 2011)
  • Experiments Research Network Second Seminar (30 March 2011)

Some current projects

The responsiveness of local politicians to citizen interest groups by Liz Richardson and Peter John

A decisive variable in effectiveness of informational lobbying is the nature and quality of information a lobbying group can offer. The aims of our citizen interest group experiment are to test out the level of responsiveness of local elected representatives to lobbying by the citizen interest groups from the voluntary, community and third sectors and to measure the level of impact of different types of lobbying.

RCT of patients entering the Expert Patients Programme (EPP)

EPP is a new approach to chronic disease management which aims to embed lay led self-management training programmes for patients with chronic conditions in the NHS. This RCT assessed clinical and cost effectiveness in terms of improvement in participants' health outcomes, cognitive symptoms, and costs of health care.

Key references

  • Designing Randomised Trials in Health, Education and the Social Sciences, by David Torgerson and Carole Torgerson, Macmillan 2008
  • Get Out the Vote: How to Increase Voter Turnout, by Donald P. Green, Alan S. Gerber, Brookings, 2008

Useful centres

PDF slides

Download PDF slides of the presentation 'What is an experiment?'