Table 1 Roles of different methods within a mixed method study

Defining the research question

A qualitative method can generate a hypothesis for a quantitative method to test [38], establish the theoretical framework for the quantitative method [39], or help conceptualise the whole study [16].

Addressing the range of research questions

Understanding how interventions work in the real world

A complex intervention may operate differently in practice from the original intention and qualitative research can address how an intervention is used in practice while quantitative research is used to measure outcomes [40]. The strength of qualitative research to assess processes has been noted in social research [38].

Getting a range of perspectives

Qualitative research can help researchers to gain access to the views of participants while quantitative research allows researchers to explore their own agenda [38].

Designing the study

Determining the sample

A quantitative method can facilitate the sampling strategy for a qualitative method [38], for example a survey can distinguish representative from non-representative cases [39].

Improving the conduct of a method

When designing a trial, qualitative research may help to design appropriate recruitment strategies and information [21]. This could be used for other quantitative methods such as surveys.

Designing study instruments

A qualitative method can help to design good survey instruments [16, 39, 41], and aid scale construction from them [38]. In the context of evaluation, it can identify the outcomes important to different stakeholders, for inclusion within instruments [32].

Developing or optimising interventions

When evaluating an intervention like a service [42], qualitative methods can help to develop the intervention [18], develop an understanding of how the intervention works and who it might be most effective for [43], and indicate why the intervention has not worked [18].

Analysis

The results from one method can affect the analysis of the other method, or qualitative and quantitative data can be combined for further understanding [44]. For example, qualitative data can be 'quantitised', that is, numerically coded for analysis with the quantitative data.

Making use of the findings

Interpreting the findings

Each method can provide different aspects of a phenomenon [38]. A qualitative method can explain factors underlying relationships in a quantitative study [38], confirm or contradict survey findings, interpret statistical relationships, explore puzzling responses or results, or offer case study illustrations [39]. It may change the interpretation of findings [32], for example urging that a treatment is not rejected as ineffective simply because it was not used, but finding a way of it being used so that it might be effective [45]. In the context of evaluation, qualitative methods can describe the context in which the study operates, in particular what is going on with controls, thus aiding interpretation [32].

Determining generalisability

A quantitative method can help to generalise a qualitative study [38], for example a survey can situate the context of case studies [1].

Implementation

Qualitative methods can be used to consider the results of a study and their application within a real world context, drawing on pluralistic views of different stakeholders [32].