We talked last week about an apparent error Supreme Court Chief Justice John Roberts made while criticizing the Voting Rights Act, and re-reading the post, I'm afraid it was more complicated than it needed to be. So, let's clarify matters a bit.
To briefly recap, Roberts, hoping to make the case against the landmark civil-rights law, pressed Solicitor General Don Verrilli on which state has "the worst ratio of white voter turnout to African-American voter turnout" and "the greatest disparity in [voter] registration between white and African American" voters. In both cases, the chief justice said, Massachusetts is the worst while Mississippi is among the best.
The underlying point was obvious: Roberts rejects the argument that Southern states, with a history of systemic discrimination and institutional racism, should be subjected to tougher scrutiny than other states. To that end, the justice was eager to argue that a Southern "red" state like Mississippi does a great job on registering and turning out African-American voters, while a Northern "blue" state like Massachusetts does an awful job.
But Roberts is relying on a dubious statistical analysis. NPR's Nina Totenberg explained this better than I did:
The Census Bureau does voting surveys to look at voting patterns nationwide, but the survey is based on a very small sample. Most recently, in 2010, the survey looked at 94,208 voters nationwide. Break that up into roughly proportional samples in each state, Census officials say, and it is really not possible to compare states because those with relatively low minority populations have a much higher margin of error.
The number of black citizens eligible to vote in Massachusetts is 236,000, while it is 721,000 in Mississippi, more than three times that number. Therefore, according to Census officials, when looking at the estimated turnout rate in Massachusetts, the voting percentage for African-Americans at first blush is estimated at 39.3 percent. But the margin of error is 11.5 percentage points, meaning that the black voter turnout actually could be as high as 50.8 percent (or, conversely, as low as 27.8 percent).
Now, look at Mississippi, where black turnout is listed at 48.7 percent. But because of the large size of the African-American population that was sampled, the margin of error is only 5.4 percentage points.
That means that factoring in the margin of error, the black turnout rate in Mississippi could be as high as 54.1 percent, or as low as 43.3 percent.
For Roberts, Mississippi clearly tops Massachusetts. But given the small sample size, it's equally plausible [update: OK, perhaps not "equally"], that Massachusetts has an African-American voter turnout rate that's 7.5% better than Mississippi's. Totenberg added, "Bottom line, as Census officials told me, these numbers are simply not reliable for state-by-state comparisons because of the high margins of error in some states."
Will Roberts walk his claim back? Probably not. Should he consider these details before striking down all or part of the landmark civil-rights legislation? Definitely.