The systematic review methodology represented by Cochrane is the mainstream both at home and abroad currently. Although its standardized procesdures have contributed to large amount of high-quality research evidence for policy-making and clinical practice, it has the unavoidable defect of poor external validity. Realist review is a theory-driven systematic review methodology for complex interventions under the realism philosophy. It does not answer the effectiveness questions but to uncover the mechanism as to how intervention works. Realist review starts with one or more candidate program theories that can explain the research question. By integrating relevant research evidence or documents, researchers look for the potential context-mechanism-outcome configurations and the recurrent patterns of program functioning, namely demi-regularities. Researchers iteratively test, refute and refine the candidate program theories, and obtain a deeper theoretical understanding of complex interventions and provide contextual recommendations for policy makers or clinical practitioners. In this paper, the characteristics of complex interventions, realism philosophy, research purpose and steps of conducting a realist review and its comparison with Cochrane systematic review are discussed in detail. Researchers should not only conduct Cochrane systematic reviews to investigate the effectiveness of interventions, but also consider realist review to explain the causal mechanism underneath complex interventions so as to provide more contextual recommendations for clinical practice. 

1. Fontelo P, Liu F. A review of recent publication trends from top publishing countries[J]. Syst Rev 2018. 7(1): 147. DOI: 10.1186/s13643-018-0819-1.

2. Greenhalgh T, Papoutsi C. Studying complexity in health services research: desperately seeking an overdue paradigm shift[J]. BMC Med, 2018, 16(1): 95. DOI: 10.1186/s12916-018-1089-4.

3. Whitty CJ. What makes an academic paper useful for health policy?[J]. BMC Med, 2015, 13: 301. DOI: 10.1186/s12916-015-0544-8.

4. Chaffee MW, McNeill MM. A model of nursing as a complex adaptive system[J]. Nurs Outlook, 2007, 55(5): 232-241. DOI: 10.1016/j.outlook.2007.04.003.

5. Greenhalgh T, Thorne S, Malterud K. Time to challenge the spurious hierarchy of systematic over narrative reviews?[J]. Eur J Clin Invest, 2018, 48(6): e12931. DOI: 10.1111/eci.12931.

6. Greenhalgh T. Outside the box: Why are Cochrane reviews so boring?[J]. Br J Gen Pract, 2012, 62(600): 371. DOI: 10.3399/bjgp12X652418.

7. Guise JM, Chang C, Butler M, et al. AHRQ series on complex intervention systematic reviews-paper 1: an introduction to a series of articles that provide guidance and tools for reviews of complex interventions[J]. J Clin Epidemiol, 2017, 90: 6-10. DOI: 10.1016/j.jclinepi.2017.06.011.

8. Campbell M, Fitzpatrick R, Haines A, et al. Framework for design and evaluation of complex interventions to improve health[J]. BMJ, 2000, 321(7262): 694-696. DOI: 10.1136/bmj.321.7262.694.

9. Craig P, Dieppe P, Macintyre S, et al. Developing and evaluating complex interventions: the new Medical Research Council guidance[J]. BMJ, 2008, 337: a1655. DOI: 10.1136/bmj.a1655.

10.    Petticrew M, Anderson L, Elder R, et al. Complex interventions and their implications for systematic reviews: a pragmatic approach[J]. J Clin Epidemiol, 2013, 66(11): 1209-1214. DOI: 10.1016/j.jclinepi.2013.06.004.

11.    Pawson R, Greenhalgh T, Harvey G, et al. Realist review--a new method of systematic review designed for complex policy interventions[J]. J Health Serv Res Policy, 2005, 10 Suppl 1: 21-34. DOI: 10.1258/1355819054308530.

12.    Pawson R, Greenhalgh T, Harvey G, et al. Realist synthesis: an introduction[M]. Manchester: ESRC Research Methods Programme, University of Manchester, 2004.

13.    Bhaskar R. A Realist Theory of Science[M]. Leeds, UK: Leeds Books, 1975.

14.    Bhaskar R. The Possibility of Naturalism[M]. Sussex, UK: Harvester Press, 1979.

15.    Bhasker R. Reclaiming reality[M]. London: Verso, 1989.

16.    Frauley J, Pearce F. Critical realism and the social sciences: Methodological and epistemological preliminaries[J]. Critical realism and the social sciences: Heterodox elaborations, 2007: 3-29. DOI: 10.3138/9781442684232.

17.    Schiller CJ. Critical realism in nursing: an emerging approach[J]. Nurs Philos, 2016, 17(2): 88-102. DOI: 10.1111/nup.12107.

18.    Pawson R, Tilley N. Realistic evaluation. Thousand Oaks, California: Sage, 1997.

19.    Pawson R, Tilley N. An introduction to scientific realist evaluation[J]. Evaluation for the 21st century: A handbook, 1997, 1997: 405-418. DOI: 10.4135/9781483348896.n29.

20.    Maxwell J A, Mittapalli K. Realism as a stance for mixed methods research[J]. Handbook of mixed methods in social & behavioral research, 2010: 145-168. DOI: 10.4135/9781506335193.n6

21.    Patomäki H, Wight C. After postpositivism? The promises of critical realism[J]. International Studies Quarterly, 2000, 44(2): 213-237.

22.    Wong G, Greenhalgh T, Westhorp G, et al. Realist methods in medical education research: what are they and what can they contribute?[J]. Med Educ, 2012, 46(1): 89-96. DOI: 10.1111/j.1365-2923.2011.04045.x.

23.    Jagosh J. Realist synthesis for public health: Building an ontologically deep understanding of how programs work, for whom, and in which contexts[J]. Annu Rev Public Health, 2019, 40: 361-372. DOI: 10.1146/annurev-publhealth-031816-044451.

24.    Wong G, Westhorp G, Pawson R, et al. Realist synthesis: RAMESES training materials[EB/OL]. (2013-07)[Access on 2020-08-01]. http://ramesesproject.org/media/Realist_reviews_training_materials.pdf.

25.    Higgins PA, Shirley MM. Levels of theoretical thinking in nursing[J]. Nurs Outlook, 2000, 48(4): 179-183. DOI: 10.1067/mno.2000.105248.

26.    Wong G, Greenhalgh T, Westhorp G, et al. RAMESES publication standards: realist syntheses[J]. J Adv Nurs, 2013, 69(5): 1005-1022. DOI: 10.1111/jan.12095.

27.    Jagosh J, Pluye P, Macaulay AC, et al. Assessing the outcomes of participatory research: protocol for identifying, selecting, appraising and synthesizing the literature for realist review[J]. Implement Sci, 2011, 6 :24. DOI: 10.1186/1748-5908-6-24.

28.    Wong G, Westhorp G, Pawson R, et al. Realist synthesis: RAMESES training materials[M]. London: University of London, 2013.

29.    Lawson T. Economics and reality[M]. London: Routledge, 1997.

30.    Meyer SB, Lunnay B. The application of abductive and retroductive inference for the design and analysis of theory-driven sociological research[J]. Sociological research online, 2013, 18(1): 1-11.

31.    Greenhalgh T, Pawson R, Wong G, et al. Retroduction in Realist Evaluation, The RAMESES II Project, 2017[EB/OL]. [Access on 2020-08-01]. http://www.ramesesproject.org/media/RAMESES_II_Retroduction.pdf.

32.    Jagosh J. Retroductive theorizing in Pawson and Tilley's applied scientific realism[J]. J Critical Realism, 2020, 19(2): 121-130. DOI: 10.1080/14767430.2020.1723301.

33.    Kristjansson EA, Robinson V, Petticrew M, et al. School feeding for improving the physical and psychosocial health of disadvantaged elementary school children[J]. Cochrane Database Syst Rev, 2007(1): Cd004676. DOI: 10.1002/14651858.CD004676.pub2.

34.    Greenhalgh T, Kristjansson E, Robinson V. Realist review to understand the efficacy of school feeding programmes[J]. BMJ, 2007, 335(7625): 858-861. DOI: 10.1136/bmj.39359.525174.AD.

35.    Rycroft-Malone J, McCormack B, Hutchinson AM, et al. Realist synthesis: illustrating the method for implementation research[J]. Implement Sci, 2012, 7: 33. DOI: 10.1186/1748-5908-7-33.

36.    Wong G, Greenhalgh T, Pawson R. Internet-based medical education: a realist review of what works, for whom and in what circumstances[J]. BMC Med Educ, 2010, 10(1): 12. DOI: 10.1186/1472-6920-10-12.

37.    Coles E, Wells M, Maxwell M, et al. The influence of contextual factors on healthcare quality improvement initiatives: what works, for whom and in what setting? Protocol for a realist review[J]. Syst Rev, 2017, 6(1): 168. DOI: 10.1186/s13643-017-0566-8.

38.    Kastner M, Estey E, Perrier L, et al. Understanding the relationship between the perceived characteristics of clinical practice guidelines and their uptake: protocol for a realist review[J]. Implement Sci, 2011, 6(1): 69. DOI: 10.1186/1748-5908-6-69.

39.    Kastner M, Bhattacharyya O, Hayden L, et al. Guideline uptake is influenced by six implementability domains for creating and communicating guidelines: a realist review[J]. J Clin Epidemiol, 2015, 68(5): 498-509. DOI: 10.1016/j.jclinepi.2014.12.013.

40.    Flynn R, Newton AS, Rotter T, et al. The sustainability of Lean in pediatric healthcare: a realist review[J]. Systematic reviews, 2018, 7(1): 137.

41.    Greenhalgh T, Robert G, Bate P, et al. How to spread ideas: A systematic review of the literature on diffusion, dissemination and sustainability of innovations in health service delivery and organisation[EB/OL]. [Access on 2020-08-01]. http://www.netscc.ac.uk/hsdr/files/project/SDO_ES_08-1201-038_V01.pdf.

42.    Pawson R, Greenhalgh T, Harvey G, Walshe K. Realist review--a new method of systematic review designed for complex policy interventions[J]. J Health Serv Res Policy, 2005, 10 Suppl 1: 21-34. DOI: 10.1258/1355819054308530.

43.    Gilmore B, McAuliffe E, Power J, et al. Data analysis and synthesis within a realist evaluation: toward more transparent methodological approaches[J]. International Journal of Qualitative Methods, 2019. DOI: 10.1177/1609406919859754.

44.    Maxwell JA. A realist approach for qualitative research[M]. Thousand Oaks, California: Sage, 2012.

45.    Maxwell JA, Miller BA. Categorizing and connecting strategies in qualitative data analysis, in Handbook of emergent methods[M]. Guilford Press: New York, 2008.

46.    Pawson R. Evidence-based policy: a realist perspective[M]. Thousand Oaks, California: Sage, 2006.

47.    Brennan N, Bryce M, Pearson M, et al. Understanding how appraisal of doctors produces its effects: a realist review protocol[J]. BMJ open, 2014, 4(6): e005466. DOI: 10.1136/bmjopen-2014-005466.

48.    Power J, Gilmore B, Vallières F, et al. Adapting health interventions for local fit when scaling-up: a realist review protocol[J]. BMJ open, 2019, 9(1): e022084. DOI: 10.1136/bmjopen-2018-022084.

49.    O'Campo P, Molnar A, Ng E, et al. Social welfare matters: a realist review of when, how, and why unemployment insurance impacts poverty and health[J]. Soc Sci Med, 2015, 132: 88-94. DOI: 10.1016/j.socscimed.2015.03.025.

50.    Best A, Greenhalgh T, Lewis S, et al. Large-system transformation in health care: a realist review[J]. The Milbank Quarterly, 2012. 90(3): 421-456. DOI: 10.1111/j.1468-0009.2012.00670.x.

51.    Goodman C, Dening T, Gordon AL, et al. Effective health care for older people living and dying in care homes: a realist review[J]. BMC Health Serv Res, 2016, 16:269. DOI: 10.1186/s12913-016-1493-4.

52.    van Hooft SM, Been-Dahmen JMJ, Ista E, et al. A realist review: what do nurse-led self-management interventions achieve for outpatients with a chronic condition?[J]. J Adv Nurs, 2017, 73(6): 1255-1271. DOI: 10.1111/jan.13189.

53.    McConnell T, Porter S. Music therapy for palliative care: A realist review[J]. Palliat Support Care, 2017, 15(4): 454-464. DOI: 10.1017/S1478951516000663.

54.    Wong G, Greenhalgh T, Westhorp G, et al. Development of methodological guidance, publication standards and training materials for realist and meta-narrative reviews: the RAMESES (Realist And Meta-narrative Evidence Syntheses – Evolving Standards) project[M]. Southampton (UK): NIHR Journals Library, 2014.