Build columns from the expected dropback model. Will return
NA on data prior to 2006 since that was before NFL started marking scrambles.
Must be run on a dataframe that has already had
clean_pbp() run on it.
Note that the functions
the database function
update_db() already include this function.
is a Data frame of play-by-play data scraped using
Additional arguments passed to a message function (for internal use).
The input Data Frame of the parameter
pbp with the following columns
Probability of dropback scaled from 0 to 1.
Dropback percent over expected on a given play scaled from 0 to 100.