Fly to Chicago HQ and had written test and one on one interview. The interview was very technical. The written test consisted of optimization, stochastic and programming language tests. Basically they tested "class" concept in C language. The department employee was nice and discussed a lot with my research and optimization algorithms.

R1 is the technical interview. R2 is an online problem-solving exam with many questions. All technical questions. They have a very high requirement for your understanding of optimizations. The questions include problems with the Linear Program/Modeling/Graph Optimization/Duals/Simplex Idea.