The article explores how to figure out the key factors for a model of short-term real interest rates. While models for nominal interest rates are well understood, those for real rates are less explored. The study estimates parameters for real interest rate models using U.S. monthly data and finds that the relationship between interest rate changes and their level is crucial. The research shows that the volatility of real interest rates changes is about half as important as for nominal rates. This suggests that a specific mathematical model works well for short-term real interest rates.