I often encounter (and use) this phrase in a context meaning to weight more heavily during decision making than is sensible, or to focus more heavily during a discussion than is warranted. For example, "You're over indexing on the one time he failed; by & large he succeeds."
What is the origin of this phrase?
This blog post talking about use of the phrase as Microsoft quotes some Google Ngrams evidence to suggest that the phrase might have originated in econometrics:
When inflation indexing was resumed in October 1983, shelter-cost increases for the 15-month period from October 1980 to December 1981 were not reflected in the updated ceiling. The indexing freeze and the 15-month indexing gap were intended to compensate for what was felt to be over-indexing of the shelter deduction ceiling prior to 1981, over-indexing brought about by the inclusion of homeowners’ costs in the CPI-U components that had been used to calculate adjustments to the ceiling.
United States Code Congressional and Administrative News, Volume 2, 1998.
In economic policy, a value is indexed to another value when it is defined to increase at the same rate as the other value. In the above case, the shelter deduction ceiling was supposed to be indexed to inflation. However, the way it was calculated was later believed to incorrect, resulting in a situation called over-indexing, where a change in the rate of inflation resulted in a outsized change in the shelter deduction ceiling because the effect of inflation was being double-counted.
