Another issue you need to take into consideration is research ethics. Depending on which country you live in, your research proposal must pass a human subject board. In other countries this is not necessary; nonetheless, there probably are data protection laws to observe. When you conduct interviews, it is mandatory that there is informed consent between you and your interviewee. This can be an oral agreement or in form of a signed document. You need to explain the purpose of your research, what you intent do with the data, who has access to the data and how long the data will be stored, and in which form the results are used and presented.
A common procedure is to make the data anonymous, . to replace all identifying information like names or persons, location and places, professional status, etc. with pseudonyms or abstract characters like A001, A002 and so on. In preparing a written consent form, pay attention to the respective data protection laws and include the legal regulation and consequences in the formulation of your text. You find examples of such forms online and also in some method books (., Helfferich, 2009).
As far as data collection goes, the “when” part of this question is relatively simple: data collection should start no later than when you begin your work – or before you begin in order to establish a baseline or starting point – and continue throughout. Ideally, you should collect data for a period of time before you start your program or intervention in order to determine if there are any trends in the data before the onset of the intervention. Additionally, in order to gauge your program’s longer-term effects, you should collect follow-up data for a period of time following the conclusion of the program.