Good morning Marianne,
This is an interesting task you have laid out here. Generally in approaching these types of questions it is helpful to start with the answer you want and then, without looking at the software, identify the data sources you have (the given data), the data that you need to create to answer the question (the derived data) and then look at the steps you will need to solve the question. Ideally you should be able to write out the process to answer to your question and then use that document to guide what software tools you use to execute the answer. Often the peculiarities of the software limit our scope and approach to solving a problem; the old saying "when all you have is a hammer everything looks like a nail" exemplifies this sort of limitation to solving problems.
A clear initial statement of the results you want will often make all the other steps of the process much easier to develop. Planning, ahead of getting into the software, will often save you considerable time and effort as compared to starting with the software and trying to figure out which tool 'may' get you to your destination. A good plan will also allow you to estimate costs and how developing this task may fit in with other tasks you may need to be working on at the same time.
Mark
