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I’m Trying To Understand Hedonic Adjustments

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Brent Donnelly is a senior risk-taker and FX market maker at HSBC New York and has been trading foreign exchange since 1995. He is the author of The Art of Currency Trading (Wiley, 2019) and his latest book, Alpha Trader, hits the shelves in Q2 2021. You can contact Brent at [email protected] and on Twitter at @donnelly_brent. As with all of our guest contributors, Brent’s post may not represent the views of Epsilon Theory or Second Foundation Partners, and should not be construed as advice to purchase or sell any security. With the intense sturm und drang around inflation right now, expect the next six months to yield intense hand-wringing and chin scratching over CPI, PCE and … hedonic quality adjustment. Quite often when people go to Twitter to rail about CPI, they

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Brent Donnelly is a senior risk-taker and FX market maker at HSBC New York and has been trading foreign exchange since 1995. He is the author of The Art of Currency Trading (Wiley, 2019) and his latest book, Alpha Trader, hits the shelves in Q2 2021.

You can contact Brent at [email protected] and on Twitter at @donnelly_brent.

As with all of our guest contributors, Brent’s post may not represent the views of Epsilon Theory or Second Foundation Partners, and should not be construed as advice to purchase or sell any security.


I’m Trying To Understand Hedonic Adjustments

With the intense sturm und drang around inflation right now, expect the next six months to yield intense hand-wringing and chin scratching over CPI, PCE and … hedonic quality adjustment. Quite often when people go to Twitter to rail about CPI, they say something like “Man, my cost of living doesn’t look like that! My tuition and health care costs are skyrocketing!” Savvy statisticians then respond with a flurry of charts that look like the one at right and say “the plural of anecdote is not data” or something similar.

So all those rising prices are being offset by all those falling prices and people are just noticing the prices that go up, right? Nope. The source of the data on that chart is the BLS, the U.S. organization charged with preparing the CPI data. The data has been sliced and diced like crazy.

The CPI index is not a cost of living measure! It is a hedonically-adjusted basket of assumptions that attempts to track some sort of cost vs. quality / total-utility metric over time. So what exactly is hedonic quality adjustment? Let’s ask the BLS:

I’m Trying To Understand Hedonic Adjustments

Hedonic quality adjustment is one of the techniques the CPI uses to account for changing product quality within some CPI item samples. Hedonic quality adjustment refers to a method of adjusting prices whenever the characteristics of the products included in the CPI change due to innovation or the introduction of completely new products.

The use of the word “hedonic” to describe this technique stems from the word’s Greek origin meaning “of or related to pleasure.” Economists approximate pleasure to the idea of utility – a measure of relative satisfaction from consumption of goods. In price index methodology, hedonic quality adjustment has come to mean the practice of decomposing an item into its constituent characteristics, obtaining estimates of the value of the utility derived from each characteristic, and using those value estimates to adjust prices when the quality of a good changes.

The CPI obtains the value estimates used to adjust prices through the statistical technique known as regression analysis. Hedonic regression models are estimated to determine the value of the utility derived from each of the characteristics that jointly constitute an item.

There is a significant degree of modelling and massaging going on. The BLS dances around the CPI as cost of living measure question here (see question #9) and the media often inaccurately describe CPI data as changes in cost of living. This leads to a situation where people see prices ripping higher and CPI at 2.0% and it makes them want to yell at CPI.

Since I always think of TVs and cars as goods that have seen flat or falling prices over the years, I decided to have a quick peek at the real life vs. BLS-imagined price movements in the world of new vehicles. I made the arbitrary and grossly simplifying decision to use the Honda Accord and the Ford Mustang as my cars because they have been popular for more than 30 years, there is plenty of historical pricing data, and their target demographics and brand image haven’t changed much over time.

Here is the Honda Accord, then and now:

I’m Trying To Understand Hedonic Adjustments

The 1990 vs. 2020 versions are clearly not the same car, especially when you compare airbags (or lack thereof), ABS, computer-assist, and all that stuff. The tricky thing for non-time-traveling humans, though, is that roughly the same socioeconomic cohort that bought Ford Mustangs or Honda Accords in 1990 is in line to buy those same cars in 2021. If you hedonically-adjust away the improvements, you are not talking about cost of living anymore, you are talking about quality-adjusted or utility-adjusted cost of living. And the result is a super-squishy approach that requires a litany of assumptions and leads to model output that is more a vague approximation of some utility-adjusted price, not an index that reflects actual real life changes in price.

The next chart is one I created that shows the evolution of the BLS new vehicle price data (red) along with the average price of a Honda Accord and a Ford Mustang each year. Funny enough, the price of a car and the average hourly wage move up at about the same speed (probably not a coincidence!). That’s how Honda Accords and Ford Mustangs remain workhorse middle class cars for over 30 years. If the price was truly remaining flat as the BLS series implies, we’d all be driving BMWs (and eventually McLarens) as wages go higher in a fairly straight line and car prices remain unch.

Did the price of cars go up, or stay flat since 1990? It’s a surprisingly difficult question to answer!

Nominal prices of cars went up about 150%. Hedonically-adjusted car prices barely moved[1]. Real car prices (adjusted for wage growth) were flat. My brain is becoming a pretzel.


I’m Trying To Understand Hedonic Adjustments

And for good order this analysis is not sensitive to what kind of car you pick. All the lines look about the same, the only difference is the upward gradient of the slope, never the direction.

And when it comes to new vehicles, here are some quality changes that might trigger a hedonic adjustment:

I’m Trying To Understand Hedonic Adjustments

If you’re curious, here is the BLS explainer for hedonic adjustment, which includes this fun stuff:

If the item being modeled is men’s shirts, the independent variables might be sleeve length and fiber composition; a simplified version of a hedonic model for men’s shirts might be:

I’m Trying To Understand Hedonic Adjustments

Here all shirts are either short sleeve or long sleeve and either cotton/poly or 100% cotton. After doing the statistical processing BLS might estimate that ß1 = 0.15 and ß2 = 0.25. This indicates that a long sleeve shirt is 15 percent more valuable than a short sleeve one and that a 100% cotton shirt is 25 percent more valuable than a cotton/poly blend shirt.

If the BLS data collector is forced to replace a short-sleeve cotton/poly shirt in the CPI sample with a long sleeve 100% cotton shirt, the CPI would adjust the price of the old item by the features in the new item, leading to a price adjustment of about 49 percent (e0.15+0.25).

If the price of the original shirt had been $20 and that of the replacement shirt $30, rather than using a $10 increase in price for that sample observation, the CPI would adjust the original shirt for sleeves and cotton content resulting in a price estimate of $29.84 (20*e0.15+0.25). This adjustment attributes most of the price difference between the shirts to the change in characteristics and an increase of only $0.16 is shown.

With 46% of the products in CPI hedonically adjusted, it’s hard to know what CPI is actually telling us. Here is a good description of the problem from the Praxis Advisory blog.

While theoretically attractive, hedonic adjustment misses a key point. In all likelihood the good purchased was for the same or slightly higher price, regardless of quality (have Lexus cars dropped in price over the last 5 years?) Consider the following example. Say the only product that Americans purchase are M&M candies—100 M&Ms in a bag that costs $1.00. Each person is limited to one bag.

Through the miracle of productivity, a way is found to fill each bag with 110 M&Ms that is now priced at $1.10. Hedonic adjustment would say that the bag really only costs $1.00 and that the CPI has not increased, since each M&M still costs a penny each. But the cost of the bag of M&Ms has gone up. And since each person must buy a bag, instead of an individual M&M, their cost of living has gone up by 10%. They must fork over an extra dime even though they’re getting more for their money. We can’t buy individual parts of a new car; we have to buy the whole car, complete with quality improvements. And the whole package costs more, improved or not.

While hedonic adjustment may accurately reflect productivity increases, they don’t accurately reflect America’s cost to live. These adjustments accrue to businesses (which in turn don’t provide adequate raises) and the federal government (which in turn under-compensates Social Security recipients).

Which is all fine and good. But we’re still left with this:


A Honda Accord cost $12,000 in 1990 and it costs $25,000 now.

A Mustang was $9,000 and now it’s $27,000.

The BLS has new car prices close to unchanged over the past 30 years.


Here’s the best my pretzel-brain can do to reconcile this. Here are my takeaways:

  • People think CPI is a cost of living index, but it’s not. Stop comparing CPI to how your cost of living has changed.
  • That said, the BLS does not push back very hard on the idea of CPI as a cost of living index!
  • Even though CPI is not a cost of living index, CPI is still used for cost-of-living adjustments to Social Security and other benefits. CPI is also used to price TIPS.
  • There is a point of abstraction at which the original object is no longer recognizable. It is possible that CPI is such an abstraction and this explains why it does not compute with the everyperson’s day-to-day experience. We are all staring at Salvador Dalí’s “Persistence of Memory” screaming “that’s not a clock!” It IS a clock. But it’s also not a clock.
  • Prices for an item can go up or down and that item’s contribution to CPI can be in the same direction, the opposite direction or zero.
  • Interestingly, hedonic adjustments only act as deflators. Say the airline crams another seat in your row, eliminates carry-on bags and otherwise makes your flight less happy and hedonic. Does the hedonically-adjusted price of your airfare increase? Nope.
  • CPI is a model output dependent on a huge string of assumptions, not necessarily a true reflection of price changes for goods and services purchased by individual consumers.
  • Just because you hear 14 scary inflation anecdotes tomorrow, don’t assume all or any of those price movements will impact CPI in the way that you expect. They might! Or they might not.

This has been my best attempt to better understand the inner workings of CPI and why it looks and smells so different from reality. There is no easy answer to this conundrum as evidenced by this long discussion on the topic on the Social Security dot gov website, but I think this article from Bloomberg on the history of CPI sums it up pretty well:


The first architects of price indexes appreciated the degree to which these numbers are nothing more than vague approximations that, precisely because they rest on such shaky foundations, can be put toward political ends.


Sounds about right. Have a plaid and/or ludicrous day.

I’m Trying To Understand Hedonic Adjustments

[1] I am fairly sure hedonic-adjustment of new vehicle prices started in 1998 / 1999 but I couldn’t find the precise answer.


Brent Donnelly is a senior risk-taker and FX market maker at HSBC New York and has been trading foreign exchange since 1995. He is the author of The Art of Currency Trading (Wiley, 2019) and his latest book, Alpha Trader, hits the shelves in Q2 2021.

You can contact Brent at [email protected] and on Twitter at @donnelly_brent.


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