14. SHARE-BASED COMPENSATION PLANS

We issue share-based awards under the following programs: our Cabot Microelectronics Corporation 2012 Omnibus Incentive Plan, as amended effective March 7, 2017 (OIP); our Cabot Microelectronics Corporation 2007 Employee Stock Purchase Plan, as Amended and Restated January 1, 2010 (ESPP); and, pursuant to the OIP, our Directors' Deferred Compensation Plan, as amended September 23, 2008 (DDCP), and our 2001 Executive Officer Deposit Share Program (DSP). In March 2017, our stockholders reapproved the material terms of performance-based awards under the OIP for purposes of complying with Section 162(m) of the Internal Revenue Code of 1986, as amended. For additional information regarding these programs, refer to Note 12 of "Notes to the Consolidated Financial Statements" included in Item 8 of Part II of our Annual Report on Form 10-K for the fiscal year ended September 30, 2018.

We record share-based compensation expense for all share-based awards, including stock option grants, and restricted stock, restricted stock unit and performance share unit ("PSU") awards, and employee stock purchase plan purchases. We calculate share-based compensation expense using the straight-line approach based on awards ultimately expected to vest, which requires the use of an estimated forfeiture rate. Our estimated forfeiture rate is primarily based on historical experience, but may be revised in future periods if actual forfeitures differ from the estimate. We use the Black-Scholes option-pricing model to estimate the grant date fair value of our stock options and employee stock purchase plan purchases. This model requires the input of highly subjective assumptions, including the price volatility of the underlying stock, the expected term of our stock options, expected dividend yield and the risk-free interest rate. We estimate the expected volatility of our stock options based on a combination of our stock's historical volatility and the implied volatilities from actively-traded options on our stock. We calculate the expected term of our stock options using historical stock option exercise data, and for stock option grants made prior to December 2017, we have added a slight premium to this expected term for employees who meet the definition of retirement-eligible pursuant to their stock option grants during the contractual term of the grant. As of December 2017, the provisions of stock option grants and restricted stock unit awards, except in certain circumstances including termination for cause, once an employee meets the retirement eligibility requirements, any remaining unvested share-based awards will continue to vest regardless of termination of service. Consequently, the requisite service period for the award is satisfied upon retirement eligibility. Therefore, we record the total share-based compensation expense upon award for those employees who have met the retirement eligibility at the grant date. The expected dividend yield represents our annualized dividend in dollars divided by the stock price on the date of grant. The risk-free interest rate is derived from the U.S. Treasury yield curve in effect at the time of grant.

The PSUs that have been awarded may be subject to downward or upward adjustment depending on the total shareholder return achieved by the Company during the particular performance period related to the PSUs, relative to the total shareholder return of an established market index. We use a third-party service provider to estimate the fair value of the PSUs at grant date by using a Monte Carlo simulation model. This model simulates the stock price movements of the Company and Index constituents using certain assumptions, including the stock price of the Company and index constituents, the risk-free interest rate and stock price volatility.

KMG awards granted subsequent to the entry into the definitive agreement, but prior to the Acquisition Date, were converted to our restricted stock units (“Replacement Award”), with vesting in three equal installments on the first three anniversaries of the original award date. If the recipient is terminated without cause or resigns with good reason during the 18 months following the Acquisition Date, the Replacement Award will vest as of such termination date in a number of shares equal to 150% of the Replacement Award.

The share-based compensation expense of $3,253 related to the Replacement Awards, including accelerated vesting, for the three months ended December 31, 2018 is included in the table below.

Share-based compensation expense for the three months ended December 31, 2018, and 2017, was as follows:

	Three Months Ended December 31,
	2018			2017
Income statement classifications:
Cost of sales	$	1,055		$	788
Research, development and technical		901			529
Selling, general and administrative		6,214			4,564
Total share-based compensation expense		8,170			5,881
Tax benefit		(1,691	)		(1,241	)
Total share-based compensation expense, net of tax	$	6,479		$	4,640

For additional information regarding the estimation of fair value, refer to Note 12 of "Notes to the Consolidated Financial Statements" included in Item 8 of Part II of our Annual Report on Form 10-K for the fiscal year ended September 30, 2018.