How does the Bureau of Labor Statistics account for shrinkflation in calculating the Consumer Price Index?

See Getting less for the same price? Explore how the CPI measures “shrinkflation” and its impact on inflation by Kari McNair of the BLS. Excerpts:

"You may have noticed recently that you’re going through a roll of paper towels at a faster clip or that there seem to be fewer tortilla chips in the bag. This isn’t your imagination! The concept is known as downsizing or shrinkflation. (We will use the term downsizing in this article.) As input costs increase and costs to create a product rise, companies can increase the list price of a good or they can offer a smaller amount of the product for the same price. So, a candy bar’s size might change from 1.6 ounces to 1.5 ounces, yet the price stays the same. In other words, the price per unit the consumer pays increases as the amount they purchase decreases, while the price they pay at the register remains the same."

"Given how easy it is to miss downsizing when you are shopping, you might be wondering if the CPI is able to reflect these types of price changes accurately. The CPI strives to capture the price change caused by downsizing through accurate data collection and effective price calculations. Our data collectors and economists identify changes to the goods and services used to calculate the CPI. Data collectors collect prices for the same unique set of goods and services over time. This includes identifying, verifying, and notifying each other of product size changes so the effective price change experienced by consumers can be accurately reflected in the CPI. When an item goes through downsizing or upsizing, the data collector reports the new data, updates the product description, and sends a message to economists in the national program office noting the product size change. Data collectors do not record information such as the number of chocolate chips in a cookie or the number of pepperonis on a pizza, however, they do record attributes such as weight and volume.

Our economists continuously review goods and services in the CPI. They identify product downsizing through monthly reviews of CPI data and online research. For products data collectors identify with a size or weight change, economists will conduct further research on the manufacturers’ websites, online shopping websites, and other sources to verify if the product is experiencing product downsizing or upsizing. Once the economist has verified that the item is experiencing product size change, they will notate the item and search the CPI sample for the same item. To ensure downsizing is captured in a timely manner, the economist will notify the data collectors that a product is experiencing downsizing so that they can be on the lookout for a size change.

Data collection procedures vary for different products and services; therefore, the impact of product size change is handled differently based on the item. An effective price per standard size, usually a price per ounce, is calculated for items where size is reported. The effective price per ounce is the collected price divided by size. For example, if a half-gallon (64 oz) of Brand A vanilla ice cream is priced in January 2021 at $5.99, then the effective price per ounce is $5.99 divided by 64 oz or $0.093 per ounce. If, in February 2021, the same Brand A vanilla ice cream is reduced in size to 60 oz, but the price is still $5.99, the effective price per ounce would be $0.0998 per ounce. This results in a 6.7-percent increase in the price per ounce of the ice cream, and the CPI would include this price increase. Our economists even adjust for items that do not have a weight, like toilet paper. For example, when the number of sheets per toilet paper roll changes from 220 per roll to 200, the economist will adjust the data to show a 10-percent price-per-sheet increase.

CPI economists track identified downsizing and upsizing in the CPI sample each month."

"product size changes (upsizing and downsizing) increased the CPI all commodity and services index by 0.01 percent per year."

"the impact of product downsizing at the all commodity and services level is minimal, with an average annual effect of 0.01 percent per year, so while consumers may notice shrinkflation at the grocery store, it has a very small impact the overall inflation picture they face."