Vertical Analysis (Common-Size) Percentages for Austin Company's Sales, Cost of Goods

Vertical analysis (common-size) percentages for Austin Company's sales, cost of goods sold, and expenses are listed here. $\begin{array}{lccc}\text { Vertical Analysis }&2015&2014&2013\\\text { Sales revenue } & 100.0 \% & 100.0 \% & 100.0 \% \\\text { Cost of goods sold } & 61.2 & 62.4 & 63.5 \\\text { Expenses } & 26.5 & 27.4 & 28.5\end{array}$ Did Austin Company's net income as a percent of sales increase, decrease, or remain unchanged over the 3-year period? Provide numerical support for your answer.