The Supreme Court yesterday heard a case about whether the US census can include a question on whether the person is a citizen, a question placed there by Wilbur Ross for the Trump administration. They’ve already lost this case in three federal courts but now it’s going to the Supreme Court. Despite the politics, there’s some important non-political arguments here that matter and aren’t being well reported. Let me highlight them.

The goal of the decennial census is an accurate count of all people. The basis of the Census comes from Article 1 Section 2 of the Constitution as amended by the 14th Amendment. The key phrase here is "counting the whole number of persons in each State". Not counting just citizens. In particular, representatives to the House are apportioned based on the entire population of each state, not just the population of citizens.

The accuracy of the count matters. It directly affects the number of House of Representatives seats and also Electoral College votes. The count also greatly influences funding apportionment, social security, etc. It’s not just statistical data for planning purposes, it is the count of record for all sorts of legislative matters.

The citizenship question would result in an undercount. The problem with the citizenship question is it will cause a lot of non-citizens to not answer the census and thus not be counted. It’s not hard to imagine why someone who is living here illegally would not want to disclose that fact to a federal officer, particularly now. But the citizenship question also discourages people who are here legally. Don’t take my word for it: the Census’ Chief Scientist John Abowd said "Three distinct analyses support the conclusion of an adverse impact on self-response and, as a result, on the accuracy and quality of the 2020 Census."

We already ask about citizenship. In addition to the decennial census, the Census Bureau also runs a fantastic demographic research program called the American Community Survey. They send detailed questionnaires of various types to a subset of American households, some every month, and collect detailed data on ethnicity, education, economics, even a count of how many people own computers. The ACS also asks about citizenship; here’s a quick view of the some of data.

We know the citizenship question will result in an undercount because of the ACS’ experience with the question. They have detailed estimates of how many people don’t answer the census at all, don’t answer the citizenship question, or quit the survey right where the citizenship question is asked. Abowd’s memo about this is long (100MB PDF), here’s a shorter relevant excerpt. Broadly speaking there’s two separate concerns; that the census taker doesn’t answer the one citizenship question, or that they don’t answer the whole census because of the presence of that one citizenship question. There’s corroborating studies from outside Census that also show the question will result in a significant undercount.

The decennial census has to be an exact count. Thanks to a 1999 Supreme Court decision the Census data used for apportionment must be an actual head count. No form of sampling or extrapolation is allowed, despite the fact that would result in a number closer to the true count. That means there’s no way to correct a census flawed with an undercount from the citizenship question.

I’ve tried to present politically neutral arguments above. How do we most accurately get an exact count of people in the US? By not adding a new question about citizenship. But of course the politics of the census can’t be ignored. The Census estimates 6.5M people may be undercounted because of this question, that’s nearly 2% of the US population. The people being undercounted tend to be Hispanic and immigrants and also tend to be pro-Democrat. Those people will be denied representation in the House and those states will be underfunded.

It’s not an accident that this question the Republican administration wants to ask just happens to help Republicans. It’s there because Steve Bannon and Kris Kobach asked Wilbur Ross to put it there, a fact that Ross apparently forgot about in earlier testimony. Ross tried to claim the Justice Department asked him to add the question but it turns out they only asked because Ross requested they ask him. The White House is now refusing to cooperate with a congressional investigation into how the question came to be on the census in the first place. There’s a detailed timeline of the political process. It’s gone much faster than the usual multi-year process for questions to carefully be added to the Census.

The Census Bureau is a competent, careful, non-partisan government agency. It’s a shame to see their work corrupted.