Patton Company warrants its products against defects for three years from date of sale. During the current year, the company made sales of \(\$ 300,000\). Store management estimated that warranty costs on those sales would total \(\$ 18,000\) over the three-year warranty period. Ultimately, the company paid \(\$ 22,000\) cash on warranties. What was the company's warranty expense for the year? What accounting principle governs this answer?