Comfort factor is the fine wool industry standard for quantifiably measuring softness: it is a measurement of the lack of fibers greater than 30 microns and should not be confused with average fiber diameter.

 

With experience handling merino wool, it's not unusual to be able to detect a 5% (or less) variation in comfort factor. Here is an interesting example of how this works out: on the test result sheet I am looking at I see one ram fleece that is 19.2 micron and it's comfort factor is 99.9% on the other hand here is another ram lamb that is 18.3 micron and it;s comfort factor is 99.7% so in this case the "coarser" fleece is actually slightly softer . The very few coarse fibers in a sea of otherwise very fine fibers have an overwhelming influence on the "prickle factor" of the finished yarn as these migrate to the surface and stick out becouse they are stiffer.  These fibers are then more likely to touch your skin as the softer more pliable fibers stay tightly twisted to the core of the yarn. The most striking example of this I have seen was when I ordered some 21.5 micron "Merino" roving from a very prominent wool importer who supplies many US retailers. The average fiber diameter may have been 21.5 micron but the comfort factor was shocking I would say about 80% - 85% at best. I suspect that this roving was made from something like bellies from fleeces that were way below 21.5 blended with some crossbreed fleeces that were coarser than 21.5.

 

Most hand spinners I have found are not yet educated as to what high comfort factor wool is (at least until they get their hands on some!) and some suppliers either don't know or aren't telling. Also the high comfort factor stuff actually requires greater skill and patience by hand spinners; therefore having that coarser fiber in the roving (sort of) serves most hand spinners needs -- just not in a softness way.