Do firms understate stock option-based compensation expense disclosed under SFAS 123?

Focusing on the four key option pricing model inputs-expected option life, expected stock price volatility, expected dividend yield, and the risk-free interest rate for the expected life of the option-this study finds that firms understate option value estimates and, thus, stock-based compensation expense disclosed under SFAS 123. As predicted based on incentives and opportunities for management to understate SFAS 123 expense, the understatement of option value estimates is increasing in proxies for the magnitude of the expense, is greater for firms with weaker corporate governance, and, to a lesser extent, is increasing in the excessiveness of executive pay. The findings are strongest for the expected option life and expected stock price volatility input assumptions, consistent with firms' greater latitude in determining these inputs. We find weaker evidence of understatement associated with the expected dividend yield assumption, and none for the interest rate assumption, consistent with these inputs being less amenable to discretion. Taken together, our findings raise some concern that the exercise of management discretion adversely affects the overall reliability of SFAS 123 expense.