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: expected life ( in years ): 0.5
volatility: -26
risk free interest rate: -1.54
2017: expected life ( in years ): 0.5
volatility: -22
risk free interest rate: -0.62
2016: expected life ( in years ): 0.5
volatility: -26
risk free interest rate: -0.37

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Questions:
what was the low rate of volatility in 2017?
what was the high rate?
what is the sum?
what is the average rate?

Answers:
22%
27%
49%
44.5%