At the time of the final analysis for progression-free survival ( median follow-up 29.6 months [ 95 % CI 27.9-36 .0 ] for the palbociclib plus letrozole group and 27.9 months [ 25.5-31 .1 ] for the letrozole group ) , 41 progression-free survival events had occurred in the palbociclib plus letrozole group and 59 in the letrozole group .