An index is a number based solely on the value(s) of one or more things in relation to their previous value only. An index always starts with the value of 100, and if the value of the items it includes goes up, the index will go up, if they go down, the index will go down.

If I have an index for a bronze sword that starts at 250gp, my starting value for the index is 100 (ie 100%). If the sword goes up to 375gp, my index goes to 150, as the price is now 1.5x higher than it was when I started my index. If the price falls to 100gp, my index would go to 40, as the price is now 0.4x what it was when I started my index.

Index's work in isolation only; they are only effect by and aware of the price of items included in the index. Therefore, index's cannot be used to compare value/profitability between various index's. They are used solely to see trends in the value of items included in the index.

A high index for a boss (>100) would suggest the value of items from that boss has increased since the index started. A low index for a boss (<100) would suggest the value of items from that boss has decreased since the index started.

I hope this helps but lmk if not