In oral arguments before the Supreme Court last week, Chief Justice John G. Roberts Jr. introduced a statistical claim that he took to imply that an important provision of the Voting Rights Act has become outmoded. Section 5 of the Voting Rights Act, which is being challenged by Shelby County, Ala., in the case before the court, requires that certain states, counties and townships with a history of racial discrimination get approval (or “pre-clearance”) from the Department of Justice before making changes to their voting laws. But Chief Justice Roberts said that Mississippi, which is covered by Section 5, has the best ratio of African-American to white turnout, while Massachusetts, which is not covered, has the worst, he said. Chief Justice Roberts’s statistics appear to come from data compiled in 2004 by the Census Bureau, which polls Americans about their voting behavior as part of its Current Population Survey. In 2004, according to the Census Bureau’s survey, the turnout rate among white voting-aged citizens was 60.2 percent in Mississippi, while the turnout rate among African-Americans was higher, 66.8 percent. In Massachusetts, conversely, the Census Bureau reported the white turnout rate at 72.0 percent but the black turnout rate at just 46.5 percent.
As much as it pleases me to see statistical data introduced in the Supreme Court, the act of citing statistical factoids is not the same thing as drawing sound inferences from them. If I were the lawyer defending the Voting Rights Act, I would have responded with two queries to Chief Justice Roberts. First, are Mississippi and Massachusetts representative of a broader trend: do states covered by Section 5 in fact have higher rates of black turnout on a consistent basis? And second, what if anything does this demonstrate about the efficacy of the Voting Rights Act?
One reason to be suspicious of the representativeness of Mississippi and Massachusetts is the high margin of error associated with these calculations, as noted by Nina Totenberg of NPR. Like other polls, the Current Population Survey is subject to sampling error, a result of collecting data among a random subsample of the population rather than everyone in the state. In states like Massachusetts that have low African-American populations, the margin of error can be especially high: it was plus-or-minus 9.6 percentage points in estimating the black turnout rate in 2004, according to the Census Bureau. Even in Mississippi, which has a larger black population, the margin of error was 5.2 percentage points.