3.6 isn't a theoretical concept, it's a form overhaul. 3.6 is the new form, a form which is supposed to present the data analysis in a credible and accurate report. A report that must be produced in a timely manner. Wasn't this the ultimate goal of 3.6? We've gone through the training, and not much has been said as to how the appraiser was actually supposed to provide the client with the completed product.
None of that relates to anything I had to say. To each their own opinion on the matter.
Our effective age comes from an analysis in which we take a significantly statical dataset (No less than 30 comparables) within a specific geographic/economic market. We then individually go through each comparable and assign an effective age based upon the observations and run it through a regression analysis program. Out of all the variables that we ran, only effective age consistently demonstrates a p-value that is relevant (<.05). We've have a couple of instances where we could use reproduce a coefficient for a garage or a bedroom, but have never successfully seen a relevant p-value from bathrooms.
That description seems to imply that you are doing a single-variable regression on each variable? Not as meaningful as multivariate regression. The other issue is that effective age, in this and almost every other approach, is being created and used as a fact, but without any basis in or weighting of how features impact prices. It transforms unrelated variables that might have dramatic differences in their impact on prices (probably related to cost) into a hard number that appears to be a measured, concrete fact.
From your other post, I would say a significant part of your problem is an insistence that only "comparable" sales be included in a regression ("to get to a homogenous data set"). Given that there is no universally agreed-upon measure or definition of what constitutes "comparable", there is no basis for that. The less variability there is among the sales you analyze, the less likely regression can perform...it is designed to explain variance, not the lack thereof. The old saying that the three most important aspects of real estate are location, location, and location is meaningful. Many appraisers are happy to go to a different area than the market area they describe as relevant to the valuation of their subject to find "comps" or derive adjustments, and ignore the difference in location, or ignore the fact that different amenities impact prices differently based on location, or simply make up a location adjustment to equate that irrelevant data to the relevant data. Efficiency is maximized by making up adjustments without analysis. Try doing multivariate regression of all or most of the sales in your subject market area. I almost always get useful results using regression in rural areas, and most of a group of 7 or 8 variables almost always have p-values of less than .05.
While the usual goal is a low p-value, it can be abused by being overly emphasized. A regression class I took 10 or 12 years ago (Regression Modeling: Why Bad Happen to Good Appraisers), they were targeting less than 10%. But, even if you are at 25%, what is the alternative? Run the model without that variable and if it makes no difference, leave it out. Otherwise, decide if an estimate, based on all the data you have available, is better than one from just 2 sales (paired) or one fabricated through depreciated cost.
As you see in this thread, there are those who have never used or tried to use regression who know all about why it can't work. There are also appraisers who have learned how it works, and when it works, and why it works, and understand how to achieve very good results with it in almost every type of market. It is funny that some who don't know what it takes suggest things like, "You really need 10 to 20 data points for everything variable.", when the usual rule of thumb by those with a clue is a minimum of about 5 datapoints for each variable in the model, with 10 to 20 being the typical preference. Yet, the same folks will make 10 adjustments based on 4 sales without any measures of reliability or significance and feel superior about their experience.
Another aspect being ignored by those without any knowledge of statistical analysis is multicollinearity. Almost every variable in real estate is related to another...number of baths or bedrooms or garages to GLA, etc. In a regression model, highly correlated variables might be indicated to be significant, even if the coefficients are not "usual". Their merit can be tested by leaving out one, then the other. But, if both are included, their "overlap" is negated. Yet, every day, appraisers are measuring adjustments from different data sets and different pairs and simply assuming there is no multicollinearity. Similarly, the relationship between GLA and prices might be linear, but it might not be. Paired sales analysis and sensitivity analysis of three sales in a table simply assume it doesn't exist. Efficiency is maximized by making up adjustments without analysis.