pre_text: Results of Continuing Operations The following table sets forth certain operational data as a percentage of net sales for the fiscal years indicated:
table: Year Ended March 31,
table: 2019 2018 2017
table: Net sales 100.0% 100.0% 100.0%
table: Cost of sales 45.2 39.2 48.4
table: Gross profit 54.8 60.8 51.6
table: Research and development 15.4 13.3 16.0
table: Selling, general and administrative 12.8 11.4 14.7
table: Amortization of acquired intangible assets 12.6 12.2 9.9
table: Special charges and other, net 0.6 0.4 2.9
table: Operating income 13.4% 23.5% 8.1%
question: Analyse this data from a financial earnings document. What was the percentage change in gross profit of net sales between 2017 and 2018?