it depends if you start a PhD in the same lab as where you worked as an RA. It is possible under these circumstances to remain on an RA salary and complete a PhD part time ie in 6yrs.. but you may have to pay your own tuition fees etc. so salary can become considerably smaller than originally thought, plus you have to earn your keep!! ie do lab work etc and be an RA for half of your time, that said, I know many who take this road and they seem really happy with their deals! I personally opted to do a PhD full time and on student stipend... but this is because I left the lab where I worked as an RA... hope this helps a bit (basically, you need to get details from the lab you aim to work in..) best of luck!