What was the change in the Beta factor in 2019 from 2018?
{"Annual growth rate (after plan period)": {"2019": 3.0, "2018": 3.0}, "Risk free rate of return": {"2019": 3.0, "2018": 3.0}, "Market risk premium": {"2019": 5.0, "2018": 4.9}, "Beta factor": {"2019": 0.83, "2018": 0.79}, "Cost of debt": {"2019": 3.3, "2018": 3.3}}
beta_factor_2019 = df["Beta factor"]["2019"]
beta_factor_2018 = df["Beta factor"]["2018"]
answer = beta_factor_2019 - beta_factor_2018