The dispute before Mr. Furman centered on Mr. Ross’s decision to add a question about citizenship to 2020 Census forms, a move that common sense suggests would lead fewer people, particularly those scared of a Trump-era, anti-immigrant backlash, to return their questionnaires. It turns out that common sense is well grounded in facts.

The Constitution requires a census every 10 years of every person, citizen and noncitizen, residing in the United States. Even small biases in the data can produce big shifts in policy and representation. With the stakes so high, Congress unsurprisingly forbade the commerce secretary from adding a question to census forms if the desired information could be gathered in some other way. In this case, citizenship information can indeed be gathered from existing administrative records. In fact, adding a question would make the data less accurate than simply relying on other records, as non-responses piled up and as noncitizen respondents lied about their status.

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Census experts figured that adding the citizenship question would cut response rates among noncitizens by 5.1 percent. A lower response rate would mean Census Bureau officials would have to follow up in person, perhaps several times, to gather information on a given household. If that did not work, the bureau would have to impute demographic information to fill the gaps in its data. The extra work would cost the government at least $27.5 million. And for all this effort and risk to the integrity of the count, the government would get nothing of value.

“In a startling number of ways,” Mr. Furman declared, “Secretary Ross’s explanations for his decision were unsupported by, or even counter to, the evidence.” The commerce secretary claimed he was simply responding to a request from the Justice Department for more information on the voting-eligible population, so as to better enforce voting-rights laws. In fact, the record shows he wanted to add the question well before Justice requested it, and he and his staff searched for help from other agencies to gin up an excuse.

The Trump administration is likely to appeal. But after Mr. Furman’s comprehensive, unassailable, 277-page drubbing, it would be astonishing if any judge allowed Mr. Ross to proceed.

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