PhD, Chemistry applicants have rated the interview process at Caltech (California) with 3 out of 5 (where 5 is the highest level of difficulty) and assessed their interview experience as 100% positive. To compare, the company-average is 75% positive. This is according to Glassdoor user ratings.
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The interview was pretty short and straightforward (only 15 minutes). They asked me why I wanted to study Chemistry, what my most significant research experience was, which project I found the most interesting. They asked me what scientific question I wanted to answer with my PhD, and they wanted to know what my goals were after graduation.