Triangulation:
As you will have realised, there are a large number of individual methods of data collection that can be used for a case study. Each have their strengths and weaknesses. It is therefore advisable to use a combination of methods - an approach known as "triangulation". As you design your study, think about how you will achieve appropriate triangulation. This can help to balance out the weaknesses of different methods, and provide a fuller picture of what is being studied.