Jani,
NJ and other states which require the pre and post mitigation results would have this information. However there is potentially a difference between results achieved by certified mitigators and non-certified mitigators. NJ did a study which predates their mandatory program which showed differences between participants in their voluntary program, 'professional' mitigators and homeowners. My recollection is that participants in their program were significantly more effective than other categories.
Larainne

Thanks for all you have done for radon Jim. Hope you enjoy your retirement.

These were high homes, so that could certainly impact it. Are the dispersion studies looking at existing homes where the foundation is surrounded by shrubs/ foundation plantings? This was in the mid-80's so plantings were well established as the homes were built in the 60's or earlier.
I agree that freezing up is an issue in Canada and the northern US. However, I think it is important to understand historically why the move away from the band/rim joist was made and the switch to mirror the plumbing exhaust took place.

The original issue with band joist exhaust was that during the EPA House Evaluation Program in an area with very high home levels - "hundreds" - we were seeing reintrainment back into the structure. You could watch the levels drop as a quick fix of extending the exhaust pipe out with a dryer hose away from the house. For these homes, this was not a theoretical issue, but a reality.

Congratulations Bill. Looking forward to using it at the Cherry Hill LUNG FORCE EXPO in March.