Mark Blumenthal | June 22, 2009
The initially released polling station data show evidence of signiﬁcant distortions in the vote counts not only for Karroubi and Rezaei but also for Ahmadinejad. No signiﬁcant distortions are apparent for Mousavi's vote counts. A key to interpreting these results is understanding why the vote counts for Karroubi and Rezaei are typically so small. Is it (a) inherently low levels of support, (b) voters strategically abandoning the candidates, or (c) fraudulent counts? If there is good reason to believe either (a) or (b), then (c) is less likely. The signiﬁcant result for Ahmadinejad is not direct proof that Ahmadinejad's votes are fraudulent, but fraud is certainly a reasonable inference in light of reports that "Iran's Guardian Council has admitted that the number of votes collected in 50 cities surpass the number of those eligible to cast ballot in those areas" (Press TV, 2009).
Andrew Gelman, who is "inclined to believe" Mebane because he is "the expert on this stuff" (I had a similar message on Thursday) also repeats this important caveat, which is consistent with Mebane's own conclusions:
[T]his sort of statistical analysis doesn't prove anything by itself, but it can be useful in giving people a sense of where to focus attention if they want to look further.