Number of IAQ Sampling Locations in Apartment Building?

I would appreciate advice on the number of sampling locations needed for LEED Testing per IEQ Credit 3.2 Option 2 in mid-rise Apartment buildings. We follow the sampling rules for commercial buildings, which are well defined, but recently we have been asked to look at residential Apartment buildings where I am finding little help on determining the number of sampling locations needed. As you know, the LEED guidelines say that samples are to be taken in each portion of the building served by a separate ventilation system …. Literally, this would mean conducting the testing in every apartment if each has its own HVAC unit! Surely this cannot be the case? A more sensible strategy would include representative locations in a percentage of the apartments, one would think? Do you know of any Credit Interpretation Rulings by LEED on this matter, or can you shed some light on the subject? Thanks in anticipation.