To calculate the inflation rate, hundreds of government workers called enumerators fan out across cities each month to check how much businesses are charging for products such as blue jeans and services such as accounting, often by visiting bricks-and-mortar stores. Statisticians roll those figures together into the consumer-price index, a data stream that shows how the cost of living is changing for typical Americans.

If the government’s enumerators can’t track down a specific price in a given city, they try to make an educated guess based on a close substitute: say, cargo pants instead of slacks. But in April, with fewer workers on hand to check prices, statisticians had to base their guesses on less comparable products or other regions of the country—a process called different-cell imputation—much more often than usual, according to the BLS