title: Systematic review of the Hawthorne effect: new concepts are needed to study research participation effects.
abstract: OBJECTIVE
  This study aims to (1) elucidate whether the Hawthorne effect exists, (2) explore under what conditions, and (3) estimate the size of any such effect.
  
  
  METHODS
  This systematic review summarizes and evaluates the strength of available evidence on the Hawthorne effect. An inclusive definition of any form of research artifact on behavior using this label, and without cointerventions, was adopted.
  
  
  RESULTS
  Nineteen purposively designed studies were included, providing quantitative data on the size of the effect in eight randomized controlled trials, five quasiexperimental studies, and six observational evaluations of reporting on one's behavior by answering questions or being directly observed and being aware of being studied. Although all but one study was undertaken within health sciences, study methods, contexts, and findings were highly heterogeneous. Most studies reported some evidence of an effect, although significant biases are judged likely because of the complexity of the evaluation object.
  
  
  CONCLUSIONS
  Consequences of research participation for behaviors being investigated do exist, although little can be securely known about the conditions under which they operate, their mechanisms of effects, or their magnitudes. New concepts are needed to guide empirical studies.
authors.affiliation: Department of Social & Environmental Health, Faculty of Public Health and Policy, London School of Hygiene & Tropical Medicine, 15-17 Tavistock Place, London WC1H 9SH, UK. Electronic address: Jim.McCambridge@lshtm.ac.uk.
authors.forename: Jim
authors.initials: J
authors.lastname: McCambridge
authors.affiliation: National Addiction Centre, Institute of Psychiatry, King's College London, 4 Windsor Walk, London SE5 8AF, UK.
authors.forename: John
authors.initials: J
authors.lastname: Witton
authors.affiliation: Department of Medical Statistics, Faculty of Epidemiology and Population Health, London School of Hygiene & Tropical Medicine, Keppel Street, London WC1E 7HT, UK.
authors.forename: Diana R
authors.initials: DR
authors.lastname: Elbourne
mesh_terms: D001519:Behavior; D006805:Human Experimentation; D006801:Humans; D016032:Randomized Controlled Trials as Topic / Q000523:psychology*; D012107:Research Design; D035842:Research Subjects / Q000523:psychology*
keywords: Assessment; Hawthorne effect; Observation; Reactivity; Research methods; Research participation