What are your most burning questions about code review?

My question is simple: What are your most burning questions about code review?

Some words on my background: I am researcher working on modern code review and its quality. We collected code review data sets from different, very smaller and very large companies. The data set does not contain the content of code review, but all meta information (who reviewed which code with how many iterations on how many files...).

I think there are even more questions that we could think of. This is why ask you - especially from a practical, not only from scientific perspective!