Hi XChen Your question is a little vague, which unfortunately makes it slightly difficult to answer. Did you have a specific exam question or part of the course in mind? I'm guessing that you may be asking about Capital Modelling. Some of the correlations that may be relevant here are discussed in Section 2.5 of Chapter 20. There may be correlations between accident years and underwriting years. For example, consider a catastrophe event that happened in early 2017 (lets say March for arguments sake). This will affect the 2017 accident year clearly, but it will also affect the 2017 underwriting year (for those policies written before the catastrophe happened) as well as any policies written in the 2016 underwriting year which were still on risk (ie exposed) when the catastrophe took place.
