This is very old news...
About 6-9 mos after the 87' stock crash, values in most of my markets began falling rapidly. I was tracking mean (MLS) sales prices in lotus, monthly by zip code. Initially I was utilizing it thru the early and mid eighties to form a basis for positive market condition adjustments.
When the market had clearly tanked and was headed south, I began making negative market condition adjustments. We lost most of our "broker" business within a few weeks when they demanded we stop making negative adjustments and we held our ground. Most if not all our competition (surprise, surprise) was calling the market flat and stable, making no adjustment for market condition.
Several very large mortgage banking operations and one bank also initially demanded we cease the negative adjustments. In three cases, I was successful in obtaining an audience with either principals, or senior reviewers/underwriters. In all three, I went from one of many appraisers on their list in my markets to their sole appraisal provider.
It was a run that lasted more than 10 years for the three clients. In all three cases, unfortunately all use AMC's now almost exclusively & the fees they offer are insulting. I provided my lotus printouts as "evidence" of market decline, in all three cases the lender asked and I obliged with copies of the monthly printouts.
Today I have almost no origination lender clients & largely service private clients who actually want to know what the property is worth, warts & all...