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. With respect to RSA and RSU grants, 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 granted 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 performance-based RSU grants tied to total shareholder return, 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 three months ended March 27, 2021, we granted 17,714 performance-based RSU awards with a grant date fair value of $131.02 per share.

Compensation cost related to RSA and RSU grants was $1.8 million and $1.0 million for the three months ended March 27, 2021 and March 28, 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 three months ended March 27, 2021:

	Shares			Weighted Average Fair Value
Balance at December 26, 2020		217,735		$	72.77
Granted		44,976		$	113.10
Vested		(24,011	)	$	63.37
Canceled		(39,256	)	$	74.30
Balance at March 27, 2021		199,444		$	82.69

As of March 27, 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 2.6 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 and $0.2 million for the three months ended March 27, 2021 and March 28, 2020, respectively, and 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 of time 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 three months ended March 27, 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		55,794		$	101.45
Exercised		(31,802	)	$	64.07
Balance at March 27, 2021		274,771		$	77.26		5.7	$	7,600,381
Options exercisable at March 27, 2021		94,618		$	73.61		3.7	$	2,991,137

As of March 27, 2021, there was $3.7 million of unrecognized compensation cost related to unvested stock options that is expected to be recognized over a weighted average period of 3.2 years.