We have a stock award and incentive program to attract and retain employees and independent directors. As of December 31, 2021, approximately 23.4 million shares were available for issuance under the Second Amended and Restated 2015 Stock Award and Incentive Plan (the “2015 Plan”). The total number of shares available for issuance under this plan may increase due to any forfeiture, cancellation, exchange, surrender, termination or expiration of an award outstanding under the 2015 Plan. Awards under the 2015 Plan may be in the form of incentive stock options, non-qualified stock options, or other types of awards as authorized under the Plan. Our plans are administered by the Compensation and Human Resources Committee of the Board.

In connection with the Separation, we entered into an agreement to modify all outstanding awards granted to the holders of such awards. Each outstanding time or performance based Aimco award was converted into one share of Aimco common stock and one share of AIR common stock. Generally, all such Aimco equity awards retain the same terms and vesting conditions as the original Aimco equity awards immediately before the Separation.

Following the Separation, compensation expense related to these modified awards for the employees retained by Aimco is incurred by Aimco. The compensation expense related to these modified awards for employees of AIR is incurred by AIR.

For the year ended December 31, 2021, total compensation cost recognized for share-based awards was (in thousands):

As of December 31, 2021, our share of total unvested compensation cost not yet recognized was $15.5 million. We expect to recognize this compensation cost over a weighted-average period of approximately 2.2 years.

We grant stock options and restricted stock awards that are subject to time-based vesting and require continuous employment, typically over a period of four to five years from the grant date, and we refer to these awards as Time-Based Stock Options and Time-Based Restricted Stock, respectively. We also grant stock options, restricted stock awards, and two forms of long-term incentive partnership units (“LTIP units”), that vest conditioned on Aimco’s total shareholder return (“TSR”), relative to identified indices over a forward-looking performance period of three years. We refer to these awards as TSR Stock Options, TSR Restricted Stock, TSR LTIP I units, and TSR LTIP II units. Vested LTIP II units may be converted at the holders’ option to LTIP Units for a conversion price over a term of 10 years. Earned TSR-based awards, if any, will generally vest 50% on each of the third anniversary and fourth anniversary of the grant date, based on continued employment. Our Time-Based Stock Options and TSR Stock Options expire generally 10 years from the date of grant.

We recognize compensation cost associated with time-based awards ratably over the requisite service periods, which are typically four to five years. We recognize compensation cost related to the TSR-based awards, which have graded vesting periods, over the requisite service period for each separate vesting tranche of the award, commencing on the grant date. The value of the TSR-based awards takes into consideration the probability that the market condition will be achieved; therefore, previously recorded compensation cost is not adjusted in the event that the market condition is not achieved, and awards do not vest.

We had Time-Based Stock Options, Time-Based Restricted Stock, TSR Stock Options, TSR Restricted Stock, TSR LTIP I units and TSR LTIP II units outstanding as of December 31, 2021. The following table summarizes activity for equity compensation for the year ended December 31, 2021.

	TSR Stock Options				Time-Based Restricted Stock Awards						TSR Restricted Stock Awards						LTIP I Units						TSR LTIP II Units
	Number of Options		Weighted Average Exercise Price		Number of Shares			Weighted Average Grant-Date Fair Value			Number of Shares			Weighted Average Grant-Date Fair Value			Number of Units			Weighted Average Grant-Date Fair Value			Number of Units		Weighted Average Exercise Price
Outstanding at beginning of year		—		—		21,473		$	43.32	(1)		16,959		$	48.58	(1)		13,752		$	53.46	(1)		8,334	$	6.12
Granted		317,200	$	6.66		1,931,997		$	6.66			269,871		$	7.98			—			—			555,556	$	4.62
Exercised		—		—		—			—			—			—			—			—			—		—
Vested		—		—		(12,345	)	$	42.52	(1)		(1,874	)	$	41.71	(1)		(1,311	)	$	51.27	(1)		—		—
Forfeited		—		—		—			—			(1,453	)	$	41.71	(1)		—			—			—		—
Outstanding at end of year		317,200	$	6.66		1,941,125		$	6.87			283,503		$	9.97			12,441		$	53.69			563,890	$	4.64

(1) Weighted average grant date fair value is based off pre-separation values when the awards were granted.

The following table summarizes the unvested or outstanding shares issued to employees of Aimco and AIR and are potentially dilutive to Aimco and Aimco Operating Partnership as of December 31, 2021.

	Unvested Shares
Awards	Aimco		AIR		Unvested Compensation Not Yet Recognized (1)
Time-Based Stock Options (Outstanding shares)		—		788,205	$	—
TSR Stock Options (Outstanding shares)		317,200		25,563		762
Time-Based Restricted Stock Awards		1,941,125		55,534		11,348
TSR Restricted Stock Awards		283,503		108,292		1,911
LTIP I units		12,441		41,007		227
TSR LTIP II units (Outstanding shares)		563,890		1,274,476		1,273
Total awards		3,118,159		2,293,077	$	15,521

We estimated the fair value of TSR-based awards granted in 2021 using a Monte Carlo simulation valuation method. Under this method, the prices of the indices and shares of our Common Stock were simulated through the end of the performance period. The correlation matrix between shares of our Common Stock and the indices as well as the corresponding return volatilities were developed based upon an analysis of historical data.

The following table includes the assumptions used for the valuation of TSR-based awards that were granted in 2021.