table of contents adobe inc .
notes to consolidated financial statements ( continued ) stock options the 2003 plan allows us to grant options to all employees , including executive officers , outside consultants and non- employee directors .
this plan will continue until the earlier of ( i ) termination by the board or ( ii ) the date on which all of the shares available for issuance under the plan have been issued and restrictions on issued shares have lapsed .
option vesting periods used in the past were generally four years and expire seven years from the effective date of grant .
we eliminated the use of stock option grants for all employees and non-employee directors but may choose to issue stock options in the future .
performance share programs our 2018 , 2017 and 2016 performance share programs aim to help focus key employees on building stockholder value , provide significant award potential for achieving outstanding company performance and enhance the ability of the company to attract and retain highly talented and competent individuals .
the executive compensation committee of our board of directors approves the terms of each of our performance share programs , including the award calculation methodology , under the terms of our 2003 plan .
shares may be earned based on the achievement of an objective relative total stockholder return measured over a three-year performance period .
performance share awards will be awarded and fully vest upon the later of the executive compensation committee's certification of the level of achievement or the three-year anniversary of each grant .
program participants generally have the ability to receive up to 200% ( 200 % ) of the target number of shares originally granted .
on january 24 , 2018 , the executive compensation committee approved the 2018 performance share program , the terms of which are similar to prior year performance share programs as discussed above .
as of november 30 , 2018 , the shares awarded under our 2018 , 2017 and 2016 performance share programs are yet to be achieved .
issuance of shares upon exercise of stock options , vesting of restricted stock units and performance shares , and purchases of shares under the espp , we will issue treasury stock .
if treasury stock is not available , common stock will be issued .
in order to minimize the impact of on-going dilution from exercises of stock options and vesting of restricted stock units and performance shares , we instituted a stock repurchase program .
see note 12 for information regarding our stock repurchase programs .
valuation of stock-based compensation stock-based compensation cost is measured at the grant date based on the fair value of the award .
our performance share awards are valued using a monte carlo simulation model .
the fair value of the awards are fixed at grant date and amortized over the longer of the remaining performance or service period .
we use the black-scholes option pricing model to determine the fair value of espp shares .
the determination of the fair value of stock-based payment awards on the date of grant using an option pricing model is affected by our stock price as well as assumptions regarding a number of complex and subjective variables .
these variables include our expected stock price volatility over the expected term of the awards , actual and projected employee stock option exercise behaviors , a risk-free interest rate and any expected dividends .
the expected term of espp shares is the average of the remaining purchase periods under each offering period .
the assumptions used to value employee stock purchase rights were as follows: .

2018
2017
2016
expected life ( in years )
0.5 - 2.0
0.5 - 2.0
0.5 - 2.0
volatility
26% ( 26 % ) - 29% ( 29 % )
22% ( 22 % ) - 27% ( 27 % )
26 - 29% ( 29 % )
risk free interest rate
1.54% ( 1.54 % ) - 2.52% ( 2.52 % )
0.62% ( 0.62 % ) - 1.41% ( 1.41 % )
0.37 - 1.06% ( 1.06 % )

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Question: what is the average volatility used to value employee stock purchase rights in 2018?
Answer: 27.5%

Question: what is the average volatility used to value employee stock purchase rights in 2017?
Answer: 44.5%