In my letter autographted by John Pistole himself, his exact words were:
If a passenger chooses to opt out of screening via an AIT machine, or sets off an alarm of one of the AIT machines or a walk-through metal detector, or is unable to proceed through either the AIT or the walk-through metal detector due to a medical condition or physical constraint, or requests private screening, that passenger will undergo a pat-down. While only a small percentage of passengers ultimately receive pat-downs - fewer than 3% - this procedure helps TSA find possible explosives, chemical weapons, and other dangerous items that otherwise might go undetected. If a passenger goes through AIT and does not alarm, in almost all instances, that person would not be subject to a pat-down. There are a very small percentage of pat-downs that are performed at random so that TSA can be unpredictable as a further deterrent to terrorists.
They've also elsewhere said that about 10% of passengers go through a NoS (although this was back before NOOD, after which NoS use seems to have significantly dropped). In addition, they've said that under 1% of pat-downs were non-NoS related. This leaves about 2%, which would mean that the TSA is admitting that 20% of NoS users receive a pat-down anyway. This would be roughly consistent with what I've seen in airports.
--Jon