In February 2019, our shareholders approved a proposal to amend our Stock Purchase Plan to increase the number of shares available under that plan by 500,000 shares. The Stock Purchase Plan as amended provides for up to 1,500,000 shares of common stock to be available for purchase by our regular employees who have completed at least one year of continuous service. In addition, there were 52,837 shares of common stock available under our 1998 Employee Stock Purchase Plan, which have been made available for issuance under our Stock Purchase Plan. The Stock Purchase Plan provides for implementation of annual offerings beginning on the first day of October in each of the years 2008 through 2027, with each offering terminating on September 30 of the following year. Each annual offering may be divided into two six-month offerings. For each offering, the purchase price per share will be the lower of: (i) 85% of the closing price of the common stock on the first day of the offering or (ii) 85% of the closing price of the common stock on the last day of the offering. The purchase price is paid through periodic payroll deductions not to exceed 10% of the participant’s earnings during each offering period. However, no participant may purchase more than $25,000 worth of common stock annually.

We used the Black-Scholes model to estimate the fair value of options granted to purchase shares issued pursuant to the Stock Purchase Plan. Volatility is based on the historical volatility of our common stock. The risk-free rate for periods within the contractual term of the options is based on the U.S. Treasury yield curve in effect at the time of grant.

As of December 31, 2022, we have issued 1,241,351 shares of common stock under our Stock Purchase Plan since its inception.