What was the percentage change in interest income between 2018 and 2019?
{"": {"2019": "", "2018": "(in thousands)", "2017": ""}, "Interest income": {"2019": 8178, "2018": 7796, "2017": 2951}, "Interest expense": {"2019": -21559, "2018": -28176, "2017": -14762}, "Other, net": {"2019": 84, "2018": -3098, "2017": 1478}, "Other expense, net": {"2019": -13297, "2018": -23478, "2017": -10333}}
interest_income_2019 = df["Interest income"]["2019"]
interest_income_2018 = df["Interest income"]["2018"]
answer = ( interest_income_2019 - interest_income_2018 ) / interest_income_2018 * 100.0