While this part is about the third section in the thesis/thesis proposal, writing your purpose statement should be job #1 in your overall process. You can’t write a proposal (well) without a clear statement of purpose.
As one of my wonderful dissertation advisers taught me, every project has both a conceptual question and an operational question. The big picture concept and the feasible one that can be actually addressed within the confines of your project. You should identify both of these before you start writing your thesis.
So what do I mean? My conceptual question could be “How do media portray breastfeeding?” This is a huge umbrella of a question and good for the overarching concept. However, if I use this question as my statement of purpose (“This study examines how media portray breastfeeding”) is is far too vague. I’ve given my reader almost no information about my actual study. This is not laying out a feasible study since I can’t study all portrayals of breastfeeding to ever exist in media.
Insert the operational question, a practical framework for the actual study I will conduct. For example, one operational question could be “How does prime-time fictional television portray breastfeeding?” Rewritten as my statement of purpose, it becomes “This study examines breastfeeding portrayals in prime-time fictional television, 1970–2011” (I did this study. Here it is: Foss. That’s not a beer bong. . . Representations of Breastfeeding. Keep in mind that this research is written as a journal article, not a thesis).
For your statement of purpose, be clear and specific about what you are doing. It should give readers an idea of the time-frame, sample, medium, and type of study (i.e. textual analysis, audience reception, experiment, you get the picture). Keep your statement to a statement. This is not the place to give us a thorough history of your sample or a lengthy explanation of how important your study is.
If you feel confused about the statement of purpose, it can help to talk to a friend about your thesis topic. What would you say? Keep in mind that your friend likely isn’t interested in a 15 minute overview of hegemony or a full report of a Pew Research study. You will get to talk about literature, background, and theory. . . .just not yet.