WASHINGTON (CNN) -- Congress is expected this week to pick apart U.S. military data suggesting attacks and civilian casualties in Baghdad have sharply decreased in recent months.
Gen. David Petraeus, left, speaks with President Bush in Iraq's Anbar province last week.
Gen. David Petraeus, the top U.S. military commander in Iraq, is scheduled to testify before Congress beginning Monday, and he is expected to tell lawmakers the troop buildup is producing results.
Petraeus will back the assertion with data indicating a lower incidence of roadside bombs and car bombs in the capital in the months leading up to the Muslim holy month of Ramadan, which begins this week.
Joined by Ryan Crocker, the U.S. ambassador to Iraq, Petraeus will begin his testimony Monday before a joint session of the House Foreign Affairs and Armed Services committees. Petraeus will address the Senate Foreign Relations Committee on Tuesday.
Petraeus spent part of Sunday at the Pentagon completing a final run through before his testimony, a source said.
Bush will soon present a report to Congress on Iraq war strategy based in part on Petraeus' and Crocker's recommendations. A senior administration official told CNN on Friday that it is "very likely" Bush will speak to the nation about Iraq in a prime-time address from the White House next week.
Meanwhile, the methodology the military is using to gauge violence in Baghdad has already come under fire.
Sen. Dick Durbin, D-Illinois, a vocal critic of the war in Iraq, has accused the White House of twisting data to suit its needs. Watch why the Pentagon says there's "no such thing" as the Petraeus Report »
"By carefully manipulating the statistics, the Bush-Petraeus report will try to persuade us that violence in Iraq is decreasing and thus the surge is working," said the Senate's No. 2 Democrat, addressing a Washington think tank last week.
Durbin's criticism was echoed by David Walker, head of the nonpartisan Government Accountability Office, who also raised questions about how the statistics were compiled.
In testimony before the Senate Armed Services Committee on Friday, Walker said he is not comfortable with the methodology used to track the death toll driven by sectarian violence.
For instance, he said, a body found with a gunshot to the front of the head is classified as an ordinary crime, while a body with a gunshot to the back of the head is attributed to sectarian violence.
Walker said he would expect such a methodology to reflect a reduction in sectarian violence.
Republican Sen. Arlen Specter -- who opposed sending additional troops to Iraq -- said Sunday he intends to examine Petraeus' testimony.
"We're going to look behind the generalizations that Gen. Petraeus or anybody gives us and probe the very hard facts to see exactly what the situation is," the Pennsylvania lawmaker said on CNN's "Late Edition."
"Unless we see some light at the end of the tunnel here, very closely examining what Gen. Petraeus and others have to say, I think there's a general sense that there needs to be a new policy."
A CNN/Opinion Research Corp. poll taken last month found about two-thirds of Americans -- 64 percent -- oppose the Iraq war, and 72 percent say even if Petraeus reports progress, that won't change their opinion.
The poll also found a great deal of skepticism about the report, with 53 percent saying they do not trust Petraeus to give an accurate assessment of the situation in Iraq.
During Friday's session, committee chairman Sen. Carl Levin, D-Michigan, said he would ask the Pentagon to declassify one of the benchmarks in the GAO's report that deals with the level of sectarian violence in Iraq.
The U.S. military data obtained by CNN indicates that 165 Iraqis were murdered in Baghdad last month, a slight increase from the previous two months. However, the number represents a significant decrease since the Baghdad security plan began earlier this year.
It is not clear how the U.S. military obtained the number, but CNN statistics -- compiled from numbers released by the Iraqi Interior Ministry -- suggest 428 Iraqis were murdered in Baghdad in August, their bodies dumped in the streets. In July, 612 Iraqis were murdered, according to the Interior Ministry.
A U.S. military chart indicates monthly casualties in Baghdad, which spiked in November at 2,200, dropped to 980 last month. The chart does not break the casualties down into deaths and injuries.
Indirect fire attacks -- a term often used for mortar and rocket assaults -- dropped from 279 in June to 146 in August, the military numbers indicate.
The data also indicate that the number of roadside bombings in the capital is dwindling, from 394 in June to 232 in August. Car bombings, including those perpetrated by suicide attackers, are returning to pre-February 2006 levels, according to the data.
Following the February 2006 bombing of the Askariya mosque in Samarra, sectarian violence spiked across the war-ravaged nation.
A separate chart provided by the military says the number of casualties from car bombs in Baghdad has "remained stable over the long term."
The military data focus only on Baghdad and do not address the increase in violence in other parts of the country since the Baghdad security plan kicked off in February.
Last month, Iraq suffered the bloodiest single attack of the war when two suicide truck bombers struck villages in northern Iraq's Nineveh province, killing at least 400 people and wounding hundreds more. Most of the casualties were members of Iraq's Yazidi religious minority. E-mail to a friend
CNN's Barbara Starr, Dana Bash and Ted Barrett contributed to this report.