Most universities require applicants to have two academic reference letters for their PhD applications. I am just wondering if the letters do affect the chance of being admitted into a PhD program as compared to academic results, education qualifications and the quality of the research proposal?

If the letters are important, does it mean that I should look for associate professors or assistant professors who taugt me before to be my referees instead of lecturers?

References can be very important. They should be from academics senior to you who have worked closely with you e.g. MSc supervisor, or at least those whom have graded your work e.g. a lecturer.

Good grades alone won't secure admission to a Ph.D. program in ... Writing samples and letters of recommendation are also very important.