mcare_pta_premium_share = if(magi <= mcare_pta_thresh1(year), mcare_ptb_base(year), if(magi <= mcare_pta_thresh2(year), mcare_ptb_bracket2(year), if(magi <= mcare_pta_thresh3(year), mcare_ptb_bracket3(year), if(magi <= mcare_pta_thresh4(year), mcare_ptb_bracket4(year), mcare_ptb_bracket5(year)) ) ) )