I provide reference letters for graduate school applications under one of the following two circumstances:

1) Worked as a research assistant in my Mobile Systems and Networking (MSN) group for at least one year.
2) Received an "A" or "A-" in my class (MATLAB / Mobile Computing).

I encourage my students to apply for Ph.D. studies in top universities. For master program applicants, I would provide each student reference letters for at most 6 universities, unless he/she has a joint publication with me in a decent journal or conference (posters and workshop papers are not counted), or has made substantial contributions to our industrial projects.