First interview was largely on fit and personality, at an academic conference. The two gentlemen I spoke with were very nice and clearly weren't there to put applicants through any sort of grinder. Several days later I had a phone interview, which consisted of a few puzzle problems. I remember being asked a question about 25 horses, and you want to find the fastest 2 or 3 or 4 (I forget) despite having no stopwatch, and only being able to race 5 horses at a time. (You wish to accomplish this in as few heats as possible.) I struggled with this problem but was able to solve it, in part because the interviewer gave me some helpful hints when I got stuck on the wrong track. An ensuing phone interview focused entirely on statistics. I was rusty and could not remember anything salient about the chi-squared distribution when it was brought up (for example).