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 851,134 shares available for issue as of September 30, 2019. As of September 30, 2019, $2,371,309 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 $1,729,025, $2,003,207, and $2,319,975, respectively. For the year ended September 30, 2019, $1,638,829 of this expense was included in selling, general and administrative expense and $90,196 was included in cost of sales. For the year ended September 30, 2018, $1,835,086 of this expense was included in selling, general and administrative expense and $168,121 was included in cost of sales. For the year ended September 30, 2017, $2,103,621 of this expense was included in selling, general and administrative expense and $216,354 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 172,000 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 108,000 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 36,000 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 6,750 stock options that were exercised using a cashless method of exercise. For the year ended September 30, 2018, there were 2,250 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 $81,728 and $75,767, respectively.
Table:
 | 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


Question: What is the change in the percentage of weighted average expected volatility from 2018 to 2019?
Answer: -5.91
Calculation: 37.77-43.68