Using the following data provided, how does one go about calculating the ex-post real interest rate for a given period? For example, what would be the ex post real interest rate between March of 1984 and March of 1985? I know how to find inflation using CPI and I know that the real interest rate = nominal - inflation. My main issue is finding the correct nominal interest rate, please be logical with your reasoning as I don't just want a solution I need to understand why.
One year interest rates: https://research.stlouisfed.org/fred2/data/GS1.txt
CPI data: https://research.stlouisfed.org/fred2/data/CPIAUCSL.txt