Well, there are a lot of reasons why, but this takedown of Dylan Matthews sums it up nicely (boldface mine):
Matthews, a fresh-out-of-Harvard writer for Ezra Klein’s Wonkblog at the Post, is one of a new breed of journalists: the young, bean-counting, charts-and-graphs-obsessed, policy geek. Mostly found on the left side of the political spectrum, their ranks include Slate’s Matt Yglesias, the New York Times’ Nate Silver, and, of course, Klein himself. They don’t usually crunch the numbers themselves (the poll-model-building Silver’s an exception). Instead they report on those that do, from a perilous perch somewhere between academic objectivity and issue advocacy.
Here’s where they run into trouble. They’re not academics—nor do they claim to be—but their job is to distill economic and poli-sci jargon into understandable, policy-relevant bite-sized chunks. And problems arise when they get it wrong—whether due to a lack of understanding of statistics, misrepresentation of the studies they cite, or, in Matthews’ case, both….
After reading that, I, and I imagine other people, wrote in privately to point out that standard deviation is a measure of dispersion that says nothing about what might have caused the scores to vary.
The last sentence from that paragraph was later redacted. “This post has been updated to clarify some of the statistical findings summarized,” reads a note at the top of the article page.
This obfuscates the problem. This was not a confusing definition that could have been written more clearly. It was completely wrong: saying a statistical measure tells us something it does not, and that a study made a finding that it did not.
All of this would be moot if Matthews were simply stating his opinion that he doesn’t like teachers’ unions….
However, Matthews and his ilk are not that kind of old school op-ed crank. They’re wonks and proud of it. They don’t use florid metaphors or tug at the heartstrings. They give you spreadsheets, pie charts, and regression tables. The numbers speak for themselves. Except sometimes when bloggers speak for them.
What pisses me off about these guys (and they’re all men, by the way) is their willingness to misuse weapons of math destruction to comfort the comfortable and afflict the afflicted, even though they often get the math wrong.