pre_text: incentive compensation cost the following table shows components of compensation expense , relating to certain of the incentive compensation programs described above : in a0millions a0of a0dollars 2018 2017 2016 charges for estimated awards to retirement-eligible employees $ 669 $ 659 $ 555 amortization of deferred cash awards , deferred cash stock units and performance stock units 202 354 336 immediately vested stock award expense ( 1 ) 75 70 73 amortization of restricted and deferred stock awards ( 2 ) 435 474 509 .
post_text: ( 1 ) represents expense for immediately vested stock awards that generally were stock payments in lieu of cash compensation . the expense is generally accrued as cash incentive compensation in the year prior to grant . ( 2 ) all periods include amortization expense for all unvested awards to non-retirement-eligible employees. .
table: In millions of dollars 2018 2017 2016
table: Charges for estimated awards to retirement-eligible employees $669 $659 $555
table: Amortization of deferred cash awards, deferred cash stock units and performance stock units 202 354 336
table: Immediately vested stock award expense<sup>(1)</sup> 75 70 73
table: Amortization of restricted and deferred stock awards<sup>(2)</sup> 435 474 509
table: Other variable incentive compensation 640 694 710
table: Total $2,021 $2,251 $2,183
question: Analyse this data from a financial earnings document. what percentage of total compensation expense in 2018 is composed of other variable incentive compensation?