Good question. I still haven't found an answer.
Besides, it's common and necessary to use some sort index from what I've read. Everybody does it around the world.
Maybe my tinfoil hat is too tight but I swear the thought occurred to me that the Clinton administration could well have cooked the index to jack up manufacturing numbers to cover the leakage of manufacturing to the Chi-coms. I am sure their focus-groups would have gone ballistic when given numbers that indicated a decline.
(I do remember columns back then accusing the Clintons of cooking economic numbers.)
I remember / read enough about "globalization" to understand that the Clintons were / are major proponents -- and it ain't just free trade agreements.
It's technology and wealth transfers to the Chi-coms and other "developing" nations. Here the word treason fits -- even without them transfering our nuke weapons to the Chi-coms.
I might add that "progressives" sixty years ago openly advocated transferring our weapons technology (esp. nukes) to the USSR -- it was too dangerous for the world for the U.S. to be the only nation with nukes, they argued.
Hmmm. What if the Clintons did jack up manufacturing numbers, any future administration trying to correct the problem could well see a "decline" in manufacturing numbers and be in big, big trouble politically.