Just 1 hour long interview, pretty standard - talk about each other's background, and then move on to the portfolio. Then Q&A. Had a few laughs during the interview, which can tell a lot about the company culture.
Intermediate Designer applicants have rated the interview process at Perkins & Will with 2 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 62.4% positive. This is according to Glassdoor user ratings.
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