What was the change in research and development expense between 2018 and 2019?
{"Stock-based compensation expense included in cost of sales": {"2019": 369, "2018": 418, "2017": 379}, "Selling, general and administrative expense": {"2019": 3889, "2018": 3989, "2017": 4063}, "Research and development expense": {"2019": 2704, "2018": 2748, "2017": 2991}, "Stock-based compensation expense included in operating expenses": {"2019": 6593, "2018": 6737, "2017": 7054}, "Total stock-based compensation expense": {"2019": 6962, "2018": 7155, "2017": 7433}, "Tax benefit for expense associated with non-qualified options, PSUs, RSUs and restricted stock": {"2019": -1659, "2018": -1432, "2017": -1699}, "Total stock-based compensation expense, net of tax": {"2019": 5303, "2018": 5723, "2017": 5734}}
rd_expense_2019 = df["Research and development expense"]["2019"]
rd_expense_2018 = df["Research and development expense"]["2018"]
answer = rd_expense_2019 - rd_expense_2018