The Federal Reserve tried to fix the financial crisis in 2008 by putting more money into banks. This made the interest rates go down in the federal funds market. The Federal Reserve then started paying interest on reserve balances to keep the rates stable. However, the rates stayed lower than expected even after this change. The researchers made a model to explain why this happened and looked at real data to see if it was true. They found that the Federal Reserve might have trouble raising the rates even with a lot of money in the system.