I’m interested in comparing ratings of 15-year-old irons to current models. Is this valid? Some older Callaway irons have very high MPF ratings, it seems like something may have changed in the MPF methodology or measurements, to shift MPF ratings.
You left off the important part of that sentence “…and how they affect playability…”. The MPF formula hasn’t changed; it’s published in Ralph’s book The Maltby Playability Factor, and if you use the formula with the published measurements for some of the clubs your talking about, you still get the same MPF.
The reason some of those old Callaway clubs end up with such a high MPF rating is that most of them have a very long C-dimension and very low vertical c.o.g. For this particular formula, that’s how you end up with high numbers. What those numbers mean is actually pretty simple: if you strike a ball off the toe or the heel of club A with a high MPF, you’ll lose less ball speed than you would with club B that has a lower MPF rating struck in the same place.
If you use two clubs, one with a very low MPF and one with a very high MPF, and strike them both in the horizontal c.o.g. and 0.1″ above the vertical c.o.g., with the same club head speed (and assuming the same weight for the purposes of this example), they’ll go the same distance.
It doesn’t necessarily mean that club A is better than B, just that it’s more “forgiving”. The MPF rating doesn’t take into account leading edge height or shape, sole width, bounce, etc. that all affect how “playable” clubs are.
The method of measuring an irons MPF has not changed. That is the great thing about it. You can truly compare the mass and dimensional characteristics of irons from one generation to the next. These mass and dimensional characteristics and how they affect playability never change. As long as you understand that you are comparing the specific mass and dimensional characteristic measurements (vertical cg, horizontal cg, rearward cg, MOI) that Ralph outlined, you can truly make comparisons.