Making comparisons with queries

I would like to generate the following report: I have
collected statistical data at 3-month intervals for 2
years. I created a lookup field to indicate the reporting
period as I continue to gather data. My question: How
do I design a query to give me results such as, "How much
more money is a person making in 3, 6, 12 mos. compared
to how much he/she was making at the initial inverview?"

I would really appreciate any help with this challenge.
Thank you.

Ellen