At the synthesis stage, results from included qualitative and quantitative studies are integrated and interpreted. Several methods for synthesizing qualitative and quantitative results have been developed. 

We conducted a review on mixed studies reviews to identify the type of synthesis designs used.  Based on typical mixed methods research designs, we found two main types of synthesis designs: sequential and convergent. The choice of the design depends on the review question(s). 

We presented the resutls of this review at the CADTH Symposium 2016 (see poster)

Reference: Hong, Q.N., Pluye, P., Bujold, M., Wassef, M. 2016 Systematic mixed studies reviews: How qualitative and quantitative evidence are synthesized? CADTH Symposium, Ottawa, 10-12 avril 2015 (poster).

In sequential exploratory synthesis, the qualitative synthesis is followed by, and informs, the quantitative synthesis. The quantitative synthesis generalizes or tests findings derived the qualitative synthesis. Hence, there are two consecutive phases: phase-one Qualitative followed by phase-two Quantitative. In phase-one (qualitative), results of qualitative, quantitative and mixed methods studies are transformed into qualitative findings (e.g., types) using qualitative thematic analysis for instance. In phase-two (quantitative), results of quantitative studies are tabulated (pooled) and compared when there is a common entity across studies (e.g., an indicator for each type). Then, the interpretation of phase-one and phase-two results suggests new hypotheses and reveals knowledge gaps. 

Example adapted from Keating et al. (2011)

Some mixed studies reviews that used a sequential exploratory synthesis: 

• Keating A, Lee A,  Holland AE. (2011). What prevents people with chronic obstructive pulmonary disease from attending pulmonary rehabilitation? A systematic review. Chronic Respiratory Disease 8(2), 89-99. 
• Mills EJ, Seely D, Rachlis B, Griffith L, Wu P, Wilson K, et al. (2006). Barriers to participation in clinical trials of cancer: a meta-analysis and systematic review of patient-reported factors. The Lancet Oncology, 7(2), 141-148.
• Pluye P, Grad RM, Dunikowski LG, Stephenson R (2005). Impact of clinical information-retrieval technology on physicians: a literature review of quantitative, qualitative and mixed methods studies. International Journal of Medical Informatics, 74(9), 745-768.
• Rosewilliam S, Roskell CA, Pandyan AD (2011). A systematic review and synthesis of the quantitative and qualitative evidence behind patient-centred goal setting in stroke rehabilitation. Clinical Rehabilitation, 25(6), 501-514.

In sequential explanatory synthesis, the quantitative synthesis is followed by, and informs, the qualitative synthesis. The qualitative synthesis helps to explain some results of the quantitative synthesis. In phase-one (quantitative), results of quantitative studies and quantitative results of mixed methods studies are tabulated (pooled in evidence tables) for instance, and the presence and importance of differences in effects are measured (e.g., using meta-analysis technique when appropriate). In phase-two (qualitative), findings of qualitative studies and qualitative findings of mixed methods studies are synthesized using qualitative thematic analysis for instance. Then, the interpretation of phase-one and phase-two results suggests new explication and reveals knowledge gaps.

Example adapted from Campbell et al. (2011)

Some mixed studies reviews that used a sequential explanatory synthesis: 

• Campbell F, Johnson M, Messina J, Guillaume L, Goyder E (2011). Behavioural interventions for weight management in pregnancy: A systematic review of quantitative and qualitative data. BMC Public Health, 11.
• Harden A, Brunton G, Fletcher A, Oakley A (2009). Teenage pregnancy and social disadvantage: systematic review integrating controlled trials and qualitative studies. British Medical Journal, 339, b4254.
• Thomas J, Harden A, Oakley A, Oliver S, Sutcliffe K, Rees R, et al. (2004). Integrating qualitative research with trials in systematic reviews. British Medical Journal, 328(7446), 1010-1012.

In convergent synthesis, results of included studies are complementary. The integration occurs for extracting and synthesizing these results using a transformation strategy. There are two categories of convergent synthesis design depending on the qualitative or quantitative nature of the data analysis.

First, there is a qualitative synthesis of results of qualitative, quantitative, and mixed methods studies (QUAL). In the data extraction for a qualitative synthesis, results of qualitative, quantitative, and mixed methods studies are transformed into themes such as constructs, concepts and factors. It is called qualitization of the data. Examples of convergent designs with qualitative synthesis: thematic synthesis, realist review, critical interpretive synthesis, meta-narrative synthesis, and multiple case synthesis. 

Second, there is a quantitative synthesis of results of qualitative, quantitative and mixed methods studies (QUAN). In the data extraction for a quantitative synthesis, results of qualitative, quantitative, and mixed methods studies are transformed into variables and values. It is called quantization of the data. Examples of convergent designs with quantitative synthesis: content analysis, Bayesian synthesis, and configurational comparative method. 

Example of convergent qualitative design adapted from Bélanger et al. (2011)

Example of convergent quantitative design adapted from Buelens et al. (2008)

Some mixed studies reviews that used a convergent qualitative synthesis: 

• Bélanger E, Rodríguez C, Groleau D (2011). Shared decision-making in palliative care: a systematic mixed studies review using narrative synthesis. Palliative Medicine, 25(3), 242-261.
• Lewis SA, Noyes J, Mackereth S, Lewis SA, Noyes J, Mackereth S (2010). Knowledge and information needs of young people with epilepsy and their parents: Mixed-method systematic review. BMC Pediatrics, 10, 103.
• Pedersen VH, Armes J, Ream E (2012). Perceptions of prostate cancer in Black African and Black Caribbean men: a systematic review of the literature. Psycho-Oncology, 21(5), 457-468.

Some mixed studies reviews that used a convergent quantitative synthesis:  

• Buelens M, Woestyne M, Mestdagh S, Bouckenooghe D (2008). Methodological issues in negotiation research: A state-of-the-art-review. Group Decision and Negotiation, 17(4), 321-345.
• Classen S, Lopez EDS (2006). Mixed methods approach explaining process of an older driver safety systematic literature review. Topics in Geriatric Rehabilitation, 22(2), 99-112.

Which synthesis design should we choose?  

We presented a workshop on this wikitool at the 2013 North American Primary Care Research Group (NAPCRG) Annual Meeting. One participant suggested the following flowchart: