A data analysis plan is a very important part of your research project that identifies and highlights the statistical techniques that you have used to examine the research questions of your project. This can be a rather difficult stage of the project, as you need to refer to how the specfic statistics you have included relate to the research questions and the kind of data that you as a researcher are gathering. It is important that what you try to do in this section of your research project is to make it clear to the reader why the statistics you have inlcluded are relevant to your hypothesis and research questions. You therefore explore the methods that you select and justify them by returning to think about the nature of your research question and hypothesis.
Thus, a data analysis plan is one where you carefully justify your statistical methods by refering back to your hypothesis and research questions.