4 empirical studies that compare the use of the flipped method

Select 4 empirical studies that compare the use of the flipped method of instruction to more traditional approaches in the context of the L2 classroom, each by (a) different author(s). For each, identify the research questions and analyze the methodology employed to address the research questions (qualitative vs. quantitative, constructs, variables, issues of validity and reliability). The studies need not be on Spanish. In the case of studies that do not focus on grammar instruction, discuss any additional constructs, variables, and/or threats to validity that would exist if the methodology were applied to grammar instruction specifically. Speculate about the modifications that could be made in order to mitigate such threats.