That was exactly my thought as soon as I read the part "...our individual behavior patterns can be erratic and unpredictable, but groups of people, even small groups, are highly predictable."! On book 3 at moment.

Although they're not publicly putting out predictions, I bet many many models have been run to see how things pan out along side reality. Advertising is just step one. Using that advertising and algorithms to push a message, step two...etc. Though that of course introduces designing the future using Psychohistory, rather than predicting whilst observing (edit to remove- "only as per the books (at least as far as I have got)") bar the instructions to the other Psychohistorians.