What was the change in interest rate between 2018 and 2019?
{"Expected life (years)": {"2019": 2.8, "2018": 2.9, "2017": 2.9}, "Interest rate": {"2019": 2.5, "2018": 2.4, "2017": 1.5}, "Volatility": {"2019": 29.3, "2018": 28.0, "2017": 26.5}, "Expected dividend yield": {"2019": "\u2014", "2018": "\u2014", "2017": "\u2014"}}
interest_rate_2019 = df["Interest rate"]["2019"]
interest_rate_2018 = df["Interest rate"]["2018"]
answer = interest_rate_2019 - interest_rate_2018