A concrete approach to user evaluation

Question

I am conducting a research in which I plan to rely on user-centered design methodology. If my understanding of this methodology is correct, it is important to have many iterations in the end of which the prototype of system is tested with users in order to make sure the developed product still meets their needs.

My time and resources are somewhat limited (I'm writing my master thesis), so I plan the research to be rather qualitative than quantative.

There are many articles on user-centered design in general, but I can't seem to find any concrete examples of the approach to user evaluation. Here are the things that I don't quite understand:

1. How many users is enough? I am aware of discount usability testing ("5 users will find about 80% of usability problems" according to Nielsen), does this apply here as well?
2. Should I hire new users for each test?
3. Is task-based evaluation adequate in my case or are there better approaches?

I do realize that these questions are nooby, but I have never tried user-centered design before.

Answer 1

For qualitative research the general consensus is that 5 is enough for each task based usability test of an iteration, but definitely recruit new participants for a new task based test of an iteration. You may have already read this but it can all be found here:

http://www.nngroup.com/articles/why-you-only-need-to-test-with-5-users/