Median progression-free survival was 10.2 months ( 95 % CI 5.7-12 .6 ) for the letrozole group and 20.2 months ( 13.8-27 .5 ) for the palbociclib plus letrozole group ( HR 0.488 , 95 % CI 0.319-0 .748 ; one-sided p = 0.0004 ) .