One thing to watch for during General Petraeus' testimony today and tomorrow: Will he continue to insist on keeping classified his command's methodology for determining which civilian attacks count as sectarian violence?

Petraeus has made numerous assertions that sectarian violence has fallen dramatically, but so far, he hasn't explained how he's derived the basis for the claim. But on Friday, the head of the Government Accountability Office, David Walker, told the Senate Armed Services Committee that Petraeus' methodology isn't something agencies throughout the government have confidence in.

Much to the chagrin of the Pentagon, the report released by Walker's GAO last week found that average daily attacks against civilians had remained flat during the lifetime of the surge. Which of those attacks qualified as sectarian violence? GAO found it wasn't clear that, contra Petraeus, sectarian attacks were on the decline, since it -- and other agencies within the government -- found the methodology for such a calculation dubious. According to the Washington Post, shooting someone in the back of the head is a sign of sectarian violence to Petraeus, but a frontal shot isn't.

I asked Multinational Force-Iraq (MNF-I), Petraeus' command, how it calculated sectarian violence last week, and I still haven't gotten a response. But here's what Walker told Sen. Jack Reed (D-RI) (the classified annex of the GAO report contains much more about Petraeus' methodology):

First, [Petraeus'] data will show that sectarian violence is going down in recent months. He will show that. Secondly, we have -- we cannot get comfortable with the methodology that's used to determine of total violence, which is sectarian and which is nonsectarian related. It's extremely difficult to do that. I mean, you know, people don't necessarily leave calling cards, you know, when certain things happen. And even if there is, you know, some type of attempt to leave information -- you don't know the accuracy or reliability of it. And so we've said that his data will show it's gone down. We're not comfortable with the methodology. And please read the classified report, because it's not just our view.

The GAO report, in draft form, said that entire agencies of the U.S. government disagreed with Petraeus' methodology on sectarian violence. Evidently, the Pentagon, and perhaps MNF-I, leaned on the GAO to shunt that statement into a classified annex. But that's exactly what Walker confirmed on Friday. Reed then challenged the basis for the classification of Petraeus' methodology and internal dissent from it:

SEN. REED: Can I ask you -- this might seem like a dumb question -- why is this classified? I mean, who are we trying to keep this information from -- the American people?

MR. WALKER: Well, we're not the ones that decide whether or not it's classified and we've expressed concerns in the past as to whether or not there's over classification of information.

SEN. REED: What difference does it make to the -- I mean, the only people who are not getting this information, frankly, are the American people and the Congress in an open session where we can honestly and fairly debate these issues with people who have access to the information and can choose to divulge what they want or not.

MR. WALKER: I would respectfully request, Senator Reed, you ought to ask the administration witnesses that, because the administration is the one that decides whether or not to classify data -- whichever administration it is -- that's not new. It's been that way for a long time.

Keep an eye on this. If Petraeus' command's methodology is sound, why not explain it, and lay the controversy to rest?