pre_text: Long-term Incentive Program Performance Share Awards During the year ended December 31, 2017, pursuant to the Company’s 2016 Incentive Plan, the Company granted long-term incentive program performance share awards (“LTIP performance shares”). These LTIP performance shares are earned, if at all, based upon the achievement, over a specified period that must not be less than one year and is typically a three-year performance period, of performance goals related to (i) the compound annual growth over the performance period in the sales for the Company as determined by the Company, and (ii) the cumulative operating income or EBITDA over the performance period as determined by the Company. Up to 200% of the LTIP performance shares may be earned upon achievement of performance goals equal to or exceeding the maximum target levels for the performance goals over the performance period. On a quarterly basis, management A summary of the nonvested LTIP performance shares is as follows: During the year ended December 31, 2019, the Company revised the expected attainment rates for outstanding LTIP performance shares due to changes in forecasted sales and operating income, resulting in additional stock-based compensation expense of approximately $3.7 million.
table: Number of Shares at Expected Attainment Weighted Average Grant Date Fair Value
table: Nonvested at December 31, 2018 540,697 $19.83
table: Forfeited -56,567 18.80
table: Change in expected attainment 185,339 20.09
table: Nonvested at December 31, 2019 669,469 $20.12
question: Analyse this data from a financial earnings document. What is the difference in Weighted Average Grant Date Fair Value between nonvested shares and forfeited shares in 2018?