Year ended September 30, 2019
Year ended September 30, 2018
Dividend yield
0%
0%
Weighted average expected volatility
37.77%
43.68%
Weighted average risk-free interest rate
2.92%
2.70%
Weighted average expected life (in years)
3.0
3.7
Vesting period (in years)
3.0
3.0
the company currently has one equity compensation plan the 2007 stock compensation plan from which it grants equity awards that are used as an incentive for directors officers and other employees .
the 2007 stock compensation plan has 851134 shares available for issue as of september 30 2019 .
as of september 30 2019 $ 2371309 of total unrecognized compensation expense related to non-vested awards is expected to be recognized over a period of approximately 4 .
9 years .
the company recorded related compensation expense for the years ended september 30 2019 2018 and 2017 of $ 1729025 $ 2003207 and $ 2319975 respectively .
for the year ended september 30 2019 $ 1638829 of this expense was included in selling general and administrative expense and $ 90196 was included in cost of sales .
for the year ended september 30 2018 $ 1835086 of this expense was included in selling general and administrative expense and $ 168121 was included in cost of sales .
for the year ended september 30 2017 $ 2103621 of this expense was included in selling general and administrative expense and $ 216354 was included in cost of sales .
stock options : the company uses the black-scholes option pricing model to determine the weighted average fair value of options granted .
during the fiscal year ended september 30 2019 the company granted employees non-qualified stock options to purchase an aggregate of 172000 shares of common stock with a weighted average contractual term of 4 years a three year vesting term and a weighted average exercise price of $ 12 .
17 .
during the fiscal year ended september 30 2018 the company granted employees non-qualified stock options to purchase an aggregate of 108000 shares of common stock with a weighted average contractual term of 4 .
7 years a three year vesting term and a weighted average exercise price of $ 13 .
37 .
there were no stock options granted during the year ended september 30 2017 .
the fair value was estimated at the grant date using the assumptions listed below : .
the expected stock price volatility is based on the historical volatility of the company ’ s stock for a period approximating the expected life .
the expected life represents the period of time that options are expected to be outstanding after their grant date .
the risk-free interest rate reflects the interest rate at grant date on zero-coupon u .
s .
governmental bonds having a remaining life similar to the expected option term .
options are generally granted at fair market values determined on the date of grant and vesting normally occurs over a three to five-year period .
however options granted to directors have a one year vesting period and a six year contractual term .
the maximum contractual term is normally six years .
shares issued upon exercise of a stock option are issued from the company ’ s authorized but unissued shares .
there were 36000 options vested during the year ended september 30 2019 and no options vested during the year ended september 30 2018 .
for the year ended september 30 2019 there were 6750 stock options that were exercised using a cashless method of exercise .
for the year ended september 30 2018 there were 2250 stock options that were exercised using a cashless method of exercise .
the intrinsic value of options exercised during the years ended september 30 2019 and september 30 2018 was $ 81728 and $ 75767 respectively .

Question: What is the model used to determine the weighted average fair value of options granted?
Answer: Black-Scholes option pricing model