devon energy corporation and subsidiaries notes to consolidated financial statements 2013 ( continued ) restricted stock awards and units restricted stock awards and units are subject to the terms , conditions , restrictions and limitations , if any , that the compensation committee deems appropriate , including restrictions on continued employment . generally , the service requirement for vesting ranges from zero to four years . during the vesting period , recipients of restricted stock awards receive dividends that are not subject to restrictions or other limitations . devon estimates the fair values of restricted stock awards and units as the closing price of devon 2019s common stock on the grant date of the award or unit , which is expensed over the applicable vesting period . performance-based restricted stock awards performance-based restricted stock awards are granted to certain members of devon 2019s senior management . vesting of the awards is dependent on devon meeting certain internal performance targets and the recipient meeting certain service requirements . generally , the service requirement for vesting ranges from zero to four years . in order for awards to vest , the performance target must be met in the first year , and if met , recipients are entitled to dividends on the awards over the remaining service vesting period . if the performance target and service period requirements are not met , the award does not vest . devon estimates the fair values of the awards as the closing price of devon 2019s common stock on the grant date of the award , which is expensed over the applicable vesting period . performance share units performance share units are granted to certain members of devon 2019s senior management . each unit that vests entitles the recipient to one share of devon common stock . the vesting of these units is based on comparing devon 2019s tsr to the tsr of a predetermined group of fourteen peer companies over the specified two- or three- year performance period . the vesting of units may be between zero and 200% ( 200 % ) of the units granted depending on devon 2019s tsr as compared to the peer group on the vesting date . at the end of the vesting period , recipients receive dividend equivalents with respect to the number of units vested . the fair value of each performance share unit is estimated as of the date of grant using a monte carlo simulation with the following assumptions used for all grants made under the plan : ( i ) a risk-free interest rate based on u.s . treasury rates as of the grant date ; ( ii ) a volatility assumption based on the historical realized price volatility of devon and the designated peer group ; and ( iii ) an estimated ranking of devon among the designated peer group . the fair value of the unit on the date of grant is expensed over the applicable vesting period . the following table presents the assumptions related to performance share units granted. .
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stock options in accordance with devon 2019s incentive plans , the exercise price of stock options granted may not be less than the market value of the stock at the date of grant . in addition , options granted are exercisable during a period established for each grant , which may not exceed eight years from the date of grant . the recipient must pay the exercise price in cash or in common stock , or a combination thereof , at the time that the option is exercised . generally , the service requirement for vesting ranges from zero to four years . the fair value of stock options on .