I did an urban study on Rhodes for my SGP. I tried to do a survey, as it is recommened for primary research. However, due to the demographics of the area not many people were willing to take the survey. So I couldn't do the survey (ok, perhaps I didn't try very hard, but still) so I mentioned in my SGP that the mere fact that it was difficult to do the survey in Rhodes proves my point and other data that Rhodes has changed to a more upper-class area.
So for me, no survey and I got 38/40.
If you're doing something similar to what I did I can send you some notes if you want. I've got census information etc.
Also, remember to refer your primary case study to another case study of a similar nature in another area. I did this second one on the Melbounre 2030 project.