Vesting of RSA and RSU grants is conditional based on continued employment or service for a specified period and, in certain circumstances, the attainment of performance goals. We retain the shares underlying the grant, and any dividends paid thereon, until the vesting conditions have been met. For time-based RSA and RSU grants, compensation cost related to the stock is recognized on a straight-line basis over the vesting period and is calculated using the closing price per share of our common stock on the grant date. For performance-based RSA grants tied to growth in adjusted pre-tax income, compensation cost related to the award is recognized over the performance period and is calculated using the closing price per share of our common stock on the grant date and an estimate of the probable outcome of the performance conditions at each reporting date. Since March 2020, we have made performance-based RSU grants that vest based on our total shareholder return ranking relative to the total shareholder return of the companies comprising the S&P Mid-Cap 400 Growth Index over a three-year performance period. For these awards, compensation cost related to the award is recognized on a straight-line basis over the performance period and is calculated using the simulated fair value per share of our common stock based on the application of a Monte Carlo simulation model. For the nine months ended September 25, 2021, we granted 17,714 performance-based RSUs with a grant date fair value of $131.02 per share.

Compensation cost related to RSA and RSU grants was $1.4 million and $1.0 million for the three months ended September 25, 2021 and September 26, 2020, respectively, and $4.5 million and $3.0 million for the nine months ended September 25, 2021 and September 26, 2020, respectively, and were included in selling, general and administrative expense in the Condensed Consolidated Statements of Operations.

The following table summarizes our RSA and RSU activity for the nine months ended September 25, 2021:

	Shares			Weighted Average Fair Value
Balance at December 26, 2020		217,735		$	72.77
Granted		76,984		$	106.23
Vested		(36,042	)	$	67.05
Canceled		(45,451	)	$	74.61
Balance at September 25, 2021		213,226		$	85.42

As of September 25, 2021, there was $10.9 million of unrecognized compensation cost related to unvested RSA and RSU grants that is expected to be recognized over a weighted average period of 2.3 years.

We expense the grant-date fair value of stock options as compensation cost on a straight-line basis over the vesting period for which related services are performed. The compensation cost charged against income was $0.3 million for each of the three months ended September 25, 2021 and September 26, 2020, and $0.9 million and $0.8 million for the nine months ended September 25, 2021 and September 26, 2020, respectively. These costs are included as selling, general and administrative expense in the Condensed Consolidated Statements of Operations.

We use the Black-Scholes option valuation model to estimate the fair value of stock options granted. Expected volatility and expected dividend yield are based on the actual historical experience of our common stock. The expected life represents the period that options granted are expected to be outstanding and was calculated using historical option exercise data. The risk-free rate was based on a U.S. Treasury security with terms equal to the expected time of exercise as of the grant date.

The following table summarizes our stock option activity for the nine months ended September 25, 2021:

	Shares			Weighted Average Exercise Price		Weighted Average Remaining Term (In years)		Aggregate Intrinsic Value
Balance at December 26, 2020		250,779		$	70.21
Granted		58,393		$	101.47
Forfeited		(9,457	)	$	79.02
Exercised		(39,794	)	$	66.11
Balance at September 25, 2021		259,921		$	77.54		5.3	$	5,228,021
Options exercisable at September 25, 2021		94,762		$	73.84		3.6	$	2,159,758

As of September 25, 2021, there was $3.0 million of unrecognized compensation cost related to unvested stock options that is expected to be recognized over a weighted average period of 2.8 years.

There were 31,462 shares and 10,735 shares purchased during the nine months ended September 25, 2021 and September 26, 2020, respectively. During the nine months ended September 25, 2021 and September 26, 2020, compensation costs were $0.7 million and $0.3 million, respectively.