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I’ve worked in the inflation field for about a quarter-century (depending on how you want to count it), and I can tell you that if you really want to start a food fight at an investment conference, mention the term ‘hedonic adjustment’ as it relates to the Consumer Price Index. Thanks to substantial counter-programming by people who want you to prefer their narrative on inflation and their inflation index, people who tend to hold to the “the government is making it up” narrative about inflation like to quote hedonic adjustment as one element of proof.

The first problem with this is that people seem to think that CPI is supposed to measure how their actual cash costs change every year. It isn’t. If you look at the price of anything, it represents a trade offered by the supplier of value for value: if you give me X dollars, I will give you the widget that paints your house in 6 hours. If you don’t think that widget is worth X dollars, then you don’t buy the widget.

But widgets change. If the same vendor offers the same widget, but thanks to improvements now will paint your house in 3 hours, and now costs Y dollars, you the buyer have the same evaluation to make except now it’s the value of a 3-hour paint job versus Y dollars instead of 6-hours versus X dollars. If you want to see how the trade changed, then you can’t just compare Y versus X. You have to compare the other side of the trade also. Or, to put it another way, the difference in price (Y-X) isn’t just due to the fact that the dollar is worth less now than it was, so that even the old version of the paint-widget would cost X’, but also because it’s a better widget. You the consumer see the price going up from X to Y, but that consists of inflation X’-X, plus quality improvement of Y-X’.

There are no two ways of looking at that. If you want to measure the change in cash outlays, just count your cash outlays. But if you’re trying to measure the change in the cost of living, then you need to try to hold the standard of living constant between measurements.

So any inflation measurement needs to account for the fact that widgets change, or it will perpetually exaggerate inflation.

Most of those adjustments are pretty straightforward. If your candy bar got 20% smaller, it’s easy to account for the additional inflation that implies. In fact most of these quality adjustments are called “quality adjustments.” It becomes a ”hedonic” adjustment when the widget has a lot of different elements that give it value. Think of a car, where having better fuel efficiency is valuable but so is an improvement in the dashboard entertainment system. When the price of the car changes, it’s much harder to figure out how much of that due to inflation (paying more to get the same stuff, X-X’ in the example above) and how much is due to the change in the components of the vehicle. Enter the econometrician, who applies fancy mathematics that you may be unsurprised to learn is called a ”hedonic regression.”

Now, just about 100% of the CPI basket is subject to quality adjustment when necessary. As I said, quality adjustment is necessary. But only a small fraction of the basket is adjusted using hedonic regression.

But it’s not even as bad as that. You hear a lot of grumbling about how “hedonic adjustment says the price of a computer is falling even though it’s staying the same or going up, so obviously inflation is really higher than the government says it is.” But you almost never hear anyone complain about hedonic adjustment to shelter. The BLS, you see, adjusts for the fact that the housing stock gets older, so that if you pay the same rent from year 1 to year 2 it actually works out to be inflation because you’re getting a slightly older apartment. The real kicker? The upward hedonic adjustment to shelter inflation comes very close to balancing the downward hedonic adjustment to computers and microwaves and things. In other words, if you outlawed hedonic adjustment it wouldn’t really change the CPI hardly at all. A 2006 paper by Johnson, Reed, and Stewart found that the “net effect of hedonics from 1999 onward…is estimated to be less than 1-hundredth of 1 percent per year, specifically +0.005 percent.”[1]

So honestly, the bottom line is that people yell about hedonic adjustment for the same reason they yell at referees. They have to yell at something when they don’t like the outcome!

Is hedonic adjustment “right?” That is, does it correctly determine how much of a price change is due to inflation and how much is due to quality changes? I can say with great certainty that it is not exactly right. It’s an estimate. Virtually every financial model is an estimate. The Black-Scholes option pricing model isn’t right either – in fact, we know that the Black-Scholes model isn’t just wrong, but it’s wrong in some very systematic ways. And yet, people continue to use Black-Scholes, because we understand the ways in which it’s not right and can adjust for it.[2]

Hedonic adjustment is also not “right.” But it’s a fair approach, and if you want to adjust the CPI by removing the downward hedonic adjustments while keeping the upward hedonic adjustments (to shelter) then you can make that adjustment mentally by just adding about +0.10% per annum to the CPI. Either way…it shouldn’t tick you off.

[1] Johnson, D.S., S.B. Reed, and K.J. Stewart. 2006. “Price Measurement in the United States: a Decade After the Boskin Report.” Monthly Labor Review (May): 10–19.

[2] One big way is that since actual market movements aren’t distributed normally, and the Black-Scholes model assumes they are, the price of options that are far out-of-the-money are systematically low. Or they would be, if we didn’t adjust for this known problem by applying a volatility smile to price out-of-the-money options.

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