Add ABC function(s)

[rparrish]

input parameters

1. a grouped data frame
2. optional quoted name of numerator and denominator fields. takes 'num' and 'den' as default
3. desired direction (higher, lower). default = 'higher'

output list:
$abc = the achievable benchmark of care
$data = original data frame with new columns

• abc_flag = 1 if a member of the ABC group