If you would back up and describe the problem you're trying to solve and what the initial data are, would probably get more useful suggestions...trying to decipher and modify code alone (particularly code that doesn't do what is wanted) is hard.
It is generally a sign the data storage isn't optimal when start to create multiple variables that are essentially identical with the exception of indexing as you have above with a:n -- all those could be placed into a loop structure if the particular indices were in the form of an indexing array. And, even better would be to use a reference to the underlying values (years, I'd guess?) inside the original array and a lookup for those instead of hardcoding "magic" numbers into the function making it totally specific to the one data set.
