Theory development in quantitive and qualitative research

From my experience, theory development for quantitive research is different from that for qualitative research. For quantitive research, the researchers first need to envisage a causal relationship between two or more variables based on the gab of existing literature, which is essential.  Then, researchers need to make sense of the causal relationship by linking the proxies or attributes of one variable with those of the other variable(s) based on the knowledge in existing literature. This process of making sense of causal relationship is regarded as theory development in quantitive research.

For qualitative research, there is no need to predict the causal relationship between two variables because the researchers do not know the which variable will have effect on which variable (in other words, the direction of the causality of the relationship is not know in the theory development section or the relationship between the variables is dynamic where the direction of the causal relationship may change depending on contingent factors). The focus of theory development in qualitative research is a) how the application of an existing well-known theory will assist researchers to investigate the unknown relationship between variables; b) why the use of a particular theory is suitable/plausible in examining the relationship between two variables in a particular context.  The justification of using a well-known theory in investigating the relationship between two variables is regarded as theory development in qualitative research.