During February 2012, our shareholders approved a proposal to amend our 2008 Employee Stock Purchase Plan (“Stock Purchase Plan”) to increase the number of shares available under that plan by 500,000 shares. During February 2018, our Board of Directors approved a proposal to amend the Stock Purchase Plan to extend it such that the final annual offering will be in 2027. The Stock Purchase Plan as amended provides for up to 1,000,000 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 up to 20 annual offerings beginning on the first day of October starting in 2008, 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. The expected term of options granted is derived from the output of the option pricing model and represents the period of time that options granted are expected to be outstanding. 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.

The following are the weighted-average assumptions used for the fiscal years ended September 30,

	2016	2017	2018
Dividend yield	0.0%	0.0%	0.0%
Risk-free interest rate	0.2%	0.7%	1.5%
Volatility	50.9%	40.9%	49.9%
Expected life	Six months	Six months	Six months

As of September 30, 2018, we had issued 860,535 shares of common stock under our Stock Purchase Plan.