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<p>Certainly, though it isn’t extremely common. I do know a couple people who have left after getting an M.S. to work for a few years and then came back to do their Ph.D. a little bit later. It does happen, just not very often. I believe most people who do it are doing it to test the waters because they haven’t decided if they really want to get the Ph.D. or not. They aren’t doing just to get the experience.</p>
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<p>No one makes the assumption that you will be continuing on to get a Ph.D. when your M.S. is finished, especially since this doesn’t happen in the majority of cases. In general, if an advisor likes you and you aren’t already signed on for a Ph.D., he/she will probably make a strong push to get you to stay. I know many professors make jokes about how M.S. students leave just as they become useful, but it has some truth to it. Professors have a vested interest in trying to keep their good students around, and pretty much all of them will make some effort to convince their M.S. students to stay.</p>
<p>Usually people do stick with their same advisor for M.S. and Ph.D., but it isn’t unheard of if they switch. After all, peoples’ interests change. I know people who have switched advisors or even tweaked their major research area a little bit between M.S. and Ph.D.</p>
<p>Going straight for a Ph.D. could mean a few different things. Some universities require you to hold a M.S. before even attempting to become a Ph.D. candidate. This practice seems to be fading somewhat. Some will let you apply directly to the Ph.D. program if you are a strong enough candidate (or get into it after a semester or two on campus). In this case, you essentially start working towards a Ph.D. straight after undergrad. The way schools handle this varies. Some will give you a M.S. along the way when you have earned it, and some will just let you go straight to Ph.D. without bothering with the M.S. It just depends on the institution.</p>
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<p>In graduate school, professors have all the pull, at least when it comes to who they work with. If you leave and come back, as long as they want to work with you, they will. Basically, when you leave, just make sure you don’t burn any bridges and you will be fine.</p>