This is directly linked to your rationale and research questions – a summary of existing relevant work, identifying the gaps (both empirical and theoretical) your project will fill.
You need to give assurances that you are not just cherry-picking your sample to confirm what you feel you already know – for instance, a proposed study on whether the media contains racist bias will be stronger if you are not just working with content from the .
You should also give details of how you will negotiate access to your sources, and the more groundwork you can do in advance the better, especially if you are working on sensitive issues or with marginalised communities.
What types of people will you hope to recruit as participants, and why?
If you are working with documents, how many and how will they be selected?