Sunday, June 21, 2009

Census Coding Errors

Census Miscoding Errors

There's a new study out showing Census data on married same-sex couples vs. married opposite-sex couples--and the results are quite surprising in how much they are alike. But Professor Dale Carpenter over at Volokh Conspiracy points out that the same-sex couples are so atypical of what we know from other studies that it is likely what is called a "miscoding error," where mistakes in recording sex on forms mean that many of those identified as same-sex married couples really aren't.

There's some history on these sort of mistakes. In my book Black Demographic Data, 1790-1860, I devote a chapter to discussing how the 1840 census data was used to prove that free blacks were more likely to be "insane or idiot" or disabled the further north you went in the U.S.--and partisans of slavery used the data to prove that freedom was bad for slaves, based on this data.

While it does appear that there was actually an increase in physically disabled free blacks just north of the border, it was not because freedom was bad for blacks. It appears that the reason was that slave owners would dump disabled slaves across the border as a way of getting rid of slaves that they would otherwise have to support. (They could not free a slave in their home state without legislative permission, and this would not be granted if the goal was to dump a disabled slave onto the county poorhouse.)

All sorts of dark conspiracies were imagined for a very long time to explain how the 1840 census data ended up with these astonishing numbers--but Patricia Cline Cohen's A Calculating People: The Spread of Numeracy in Early America contains what I consider by far the most satisfying explanation--miscoding errors. The column for "idiot and insane" whites was right next to the "idiot and insane" column for blacks--and these columns were very, very long. It was therefore very easy for census marshals to accidentally enter the "idiot and insane white" count in the "idiot and insane Negro" column. Especially the further north you went, the fewer blacks there were in the population--so even moving one or two mentally disabled whites into the back column would have very disproportionate influence: the mentally ill and retarded whites who might be 1% of the white population in a small town magically became 10% of the black population.

Since homosexuals are only about 2-3% of the total U.S. population, opposite-sex married couples will vastly exceed same-sex married couples. It would only take a small number of similar miscoding errors--turning John and Janet into John and Bob--to make the same-sex married population look much more like the opposite-sex married population than it actually is. At the same time, the small number of same-sex couples means that the total miscoding errors that convert John and Bob into John and Janet will be a tiny fraction of the opposite direction. Link

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