The Company recognizes compensation cost for stock options awarded to employees based on their grant-date fair value. The value of each stock option is estimated on the date of grant using the Black-Scholes option-pricing model. The weighted-average fair value per share for the stock options granted to employees for the years ended December 31, 2018, 2017 and 2016 were $21.00, $17.71, and $15.17, respectively.

The following assumptions were used in determining the fair value of stock options for grants issued in 2018, 2017 and 2016:

The Company does not have any publicly traded stock options; therefore, expected volatilities are based on historical volatility of the Company’s stock. The risk-free interest rate is based on the yield of a zero-coupon U.S. Treasury bond whose maturity period approximates the option’s expected term. The expected life assumption represents the weighted-average period of time that newly granted stock-based awards are expected to remain outstanding. The expected life is estimated by analyzing three components of historical grants with the same vesting schedules: (i) observed post-vesting forfeiture, (ii) observed exercise behavior, and (iii) expected exercise behavior. The expected time to early exercise is calculated by assuming that the options outstanding as of the valuation date will be exercised at the midpoint between the final vest date and the expiration date. If a grant is already fully vested, it is assumed the outstanding options exercise at the midpoint between the valuation date and the expiration date. The three components are then option-weighted to estimate expected life. The Company stratifies its employees as Board of Directors, Named Executives and all other employees, each group with its own exercise behavior and thus, expected life.

For 2018, 2017, and 2016, the Company recorded $4,288, $4,368 and $3,816, respectively, in selling, general and administrative expenses for stock options. As of December 31, 2018, the total compensation cost related to unvested stock option awards granted to employees but not yet recognized was $9,871. The cost will be amortized on a straight-line basis over the remaining weighted-average vesting period of 2.7 years.

The following table is a summary of the Company’s stock option activity issued to employees and related information:

				Weighted Average
	Number of Shares			Exercise Price		Options Exercisable
Outstanding at December 31, 2017		1,484,914		$	32.53		727,645
Granted		231,455			53.61
Exercised		(599,149	)		23.43
Forfeited or expired		(65,597	)		47.55
Outstanding at December 31, 2018		1,051,623			41.41
Exercisable at December 31, 2018		450,110		$	34.40

The aggregate intrinsic value for all stock options exercised for the years ended December 31, 2018, 2017 and 2016 was $19,976, $13,775 and $14,832, respectively. The aggregate intrinsic values for all stock options outstanding and exercisable as of December 31, 2018 were $3,314 and $3,211, respectively.

A summary of the Company’s nonvested stock options, restricted stock and performance shares activity is presented below:

	Nonvested
	Stock Options					Restricted Stock					Performance Shares
	Number of Shares			Weighted- Average Grant-Date Fair Value		Number of Shares			Weighted- Average Grant-Date Fair Value		Number of Shares			Weighted- Average Grant-Date Fair Value
Nonvested at December 31, 2017		757,269		$	15.20	265			$	44.23		276,250		$	42.08
Granted		231,455			21.00		9,779			53.70		71,750			50.49
Vested during period		(321,614	)		13.84		(10,044	)		53.45		(46,000	)		41.36
Forfeited		(65,597	)		18.22		–			–		(150,000	)		41.73
Nonvested at December 31, 2018		601,513		$	17.83		-		$	-		152,000		$	46.61

Members of the Cambrex Board of Directors currently participate in an incentive plan which rewards service with restricted stock units. Awards are made annually and vest over six months. On the six month anniversary of the grant, restrictions on sale or transfer are removed and shares are issued to the Directors. These awards are classified as equity awards.

For 2018, 2017 and 2016, the Company recorded $533, $571 and $489, respectively, in selling, general and administrative expenses for restricted stock units. As of December 31, 2018, there was no compensation cost related to unvested restricted stock not yet recognized.

The Company granted equity-settled performance shares (“PSs”) to certain executives. PS awards provide the recipient the right to receive a certain number of shares of the Company’s common stock in the future, which depends on the Company’s level of achievement of net revenue and EBITDA growth as compared to the net revenue and EBITDA growth of the members of a specified peer group of companies over a three year period. The peer group consists of publicly-traded life sciences companies competing in the same industry as the Company. For 2018, 2017 and 2016, the Company recorded a benefit of $727, expense of $3,975 and expense of $3,461, respectively, in selling, general and administrative expenses related to these PS awards. As of December 31, 2018, total compensation cost related to unvested performance shares not yet recognized was $4,322. The cost will be amortized on a straight-line basis over the remaining weighted-average vesting period of 1.7 years.