Vertical analysis percentages from Waubon Corp.'s income statement are listed here:

___________________________2018...................2017...............2016

Net sales............................100.0%...............100.0%............100.0%

Cost of goods sold..............59.4%.................60.5%..............60.0%

Operating expenses.............19.6%.................20.4%..............20.0%

Income tax expense..............4.2%...................3.8%................4.0%

Assuming that Waubon did not have any non-operating or irregular items, did its net income as a percentage of sales increase, decrease, or remain unchanged over the three-year period? Explain.