How do they get calculated?

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The CPI consists of a family of indexes that measure price change experienced by urban consumers. Specifically, the CPI measures the average change in price over time of a market basket of consumer goods and services. The market basket includes everything from food items to automobiles to rent. The CPI market basket is developed from detailed expenditure information provided by families and individuals on what they actually bought. There is a time lag between the expenditure survey and its use in the CPI. For example, CPI data in 2023 was based on data collected from the Consumer Expenditure Surveys (CE) for 2021. That year, over 20,000 consumer units from around the country provided information each quarter on their spending habits in the interview survey. To collect information on frequently purchased items, such as food and personal care products, approximately another 12,000 consumer units kept diaries listing all items they bought during a 2-week period that year. This expenditure information from weekly diaries and quarterly interviews determines the relative importance, or weight, of the item categories in the CPI index structure.

The CPI represents all goods and services purchased for consumption by the reference population (U or W). BLS has classified all expenditure items into more than 200 categories, arranged into eight major groups (food and beverages, housing, apparel, transportation, medical care, recreation, education and communication, and other goods and services). Included within these major groups are various government-charged user fees, such as water and sewerage charges, auto registration fees, and vehicle tolls.

If you want to see the current makeup of the basket of goods whose prices are tracked, and their weights in the index, here is Table 1 of the most recent report. And if you want to follow the price of a specific category over time, the Federal Reserve Bank of St. Louis keeps a really helpful interactive chart service for almost every public economic stat. Here is Table 1 of the CPI report.

It's a lot of data collection on prices across a lot of transactions, and a lot of list prices, and a lot of locked in contract prices, to determine how much people are spending on different types of things, whether the quality of those things is changing over time, and what percentage of a typical household income gets spent on those types of things.