A common problem in insurance is to predict the amount (number, severity, or aggregate total) of losses in the next year for a book of business (eg. all personal auto policies). To do this, one can look at the historical correlation between past years and use that to predict the next year. For example, a co-variance matrix between years can be used to perform simple OLS regression analysis.
Question: To determine the correlation between years, you can look at the sum total for each year. Let's say you have more detailed data (by claim or by policy) and each year, some policies will stay, some will leave, and some new ones will join. Can you use this to get a better estimate, or is the sum total the best you can do?