I have been getting this question a lot in the past few weeks. A PhD and an MS are two very different things. Even though the eligibility criteria (usually specified in terms of minimum GRE & TOEFL scores and minimum GPA) are the same for the both, that’s where almost all the similarity ends. Let me break down the differences that I think are important:
• Research: The biggest difference is what you are actually expected to contribute in terms of research.
A PhD or a Doctor of Philosophy was originally awarded to a learned individual who had achieved the approval of his peers and who had demonstrated a long and productive career in the field of his/her choice. It indicated that this individual had dedicated his life to learning and to the acquisition and spread of knowledge. I can safely argue that the expectations of a PhD aspirant are not very different in today’s world. You are expected, as a PhD candidate, to come up with an original idea in your field and carry out novel research based on that idea. To this end, you are required to produce and defend a thesis or a dissertation that documents your idea, all your experiments towards the exploration of that idea and the conclusions that you reached. Remember that the keyword here is original.
For an MS, the research contribution is usually substantially lower. As an MS student, you might also be required to write an MS thesis but that does not necessarily need to be inspired by original research. You are allowed to, for example, take an already existing idea and propose some modifications or revisions to it.
• Qualifying examination: Another important difference, partially related to the point I made above, is in the official requirements of the degree. The coursework for an MS and a PhD is usually comparable, but for a PhD, you need to pass a qualifying examination that usually tests your knowledge of your research area. Qualifying examinations or quals may be written or oral. The written examinations are quite comprehensive and thorough. You are usually given 2 chances to pass these examinations. Oral examinations usually take the form of Q&A session where you answer the questions put to you by a committee.
On the other hand, for an MS, all you usually need to do is take the prescribed amount of coursework and write a thesis. As I said, the amount of coursework involved in an MS might be comparable to that for a PhD, depending on the program requirements.
• Admission Criteria: The eligibility criteria may be the same for both the applications but an admissions committee will consider different factors when choosing PhD students, as opposed to MS students. It is very difficult, if not impossible, to say what these factors may be in either case. All I can tell you is that when you are applying for a PhD, your aptitude for research is the primary factor. Indicators for this aptitude may include prior research experience, technical publications and letters of recommendation from faculty members at your undergraduate institution. Of course, the same aptitude will be a factor when you apply for an MS but I believe that it might not be the primary one. Other things such as academic credentials, work experience and GRE scores might be weighted higher in that case.
• Employment: I was once told my by advisor that the PhD degree holders are the ones that actually solve a problem and MS degree holders are the ones that implement that solution. I don’t quite agree with such a black-and-white demarcation of responsibilities, but I think it a decent approximation to the real world. If you graduate with an MS, your primary responsibilities will include implementation and development as opposed to solving problems or coming up with new ideas. I think an example always illustrates this distinction better. My former roommate graduated with an MS in Computer Engineering and his research was in Computer Architecture. He has now been at the AMD Design Validation & Verification team for 2.5 years and works on verification of processor designs. It does not entail a lot of original research but it is certainly not just programming, either. He has to use concepts from VLSI design, data structures and computer architecture and apply them to his work. The actual processor design, however, was conceived by a team of scientists whose members, most probably, hold PhDs in their respective fields.
