You are first asked to complete a general intelligence test and then invited for an assessment day. At the assessment day, they have you complete a similar but harder intelligence test and then work on three open-ended problems (the same that are mentioned in other reviews). You are then interviewed by an examiner who asks you to elaborate and explain your answers to the open-ended problems, as well as inquiring into your general interests and why you want to work for the company. While the open-ended problems are not excessively difficult, you are given a VERY small amount of time to complete them. What matters the most, however, is how you explain your answers during the interview, as it almost seems like the interviewer doesn't read your answers beforehand. What made my experience negative was that my interviewer was uninterested and dismissive. He had trouble following my mathematics-based approach to the problems (I have a maths background), but preferred to assume that my solutions were dissatisfactory instead of trying to understand them. My advice is to come up with answers that are simple and easy to explain, even if they are not theoretically tight enough to close the problem.