James_H, on 2020-June-29, 12:03, said:

I'm looking for the exact BBO calculates IMPS

I notice that it has the neat property that the IMPS sum to zero which does not always happen with cross imps.Wondering how the calculation comes up with partial IMPS

It's just cross-imps. For each table's score, IMP compare vs every other score. Add to get total. Divide by number of comparisons to get an average.

The IMPS should sum to zero when done this way. (As opposed to "Butler", the antiquated way of doing IMP pair scoring based on IMP comparing vs the "datum", average of bridge raw scores sometimes with dropping of extremes. Butler was only necessary with scoring without computer help, there's no good reason to use it anymore IMO).