EVERYTHING IN ITS SEASON....I've gotten a couple of emails asking me for more detail about the seasonality of violence in Iraq. There are several ways to show this graphically, but all of them present difficulties because the various data series aren't available in a consistent form for the entire course of the war.
That said, here's a chart that shows the seasonality pretty well. It comes via reader Thomas J., who has graphed U.S. troop deaths per thousand soldiers, a data series that's (a) available for every month since the beginning of the occupation, (b) highly consistent, and (c) a fairly decent proxy for the overall level of violence. The "per thousand soldiers" correction helps to control for the fact that troop fatalities will naturally be higher whenever there are more soldiers deployed in Iraq. This doesn't necessarily mean the level of violence is higher, just that the population at risk is higher.
The seasonality is pretty easy to see: violence peaks in spring, then declines during summer, peaks again in fall, and drops during winter. The peaks are probably overstated slightly thanks to unusually deadly April and November months in 2004, but even without that the seasonality is fairly pronounced. Roughly speaking, July troop casualties are typically about 40% lower than their April peak, whereas this year they were only about 30% lower. In other words, violence was worse than usual. On the other hand, August looked a little better than usual this year compared to 2007's spring peak. As always, what this means is that you can't draw any dramatic conclusions based on a couple of data points. However, taken as a whole the evidence pretty strongly suggests that the surge hasn't had any effect at all on overall violence levels. It's just moving in its usual seasonal pattern.
The obvious followup would be a similar chart showing the seasonality of civilian casualties. Unfortunately, there's simply no reliable data series for civilian casualties over the course of the war, and the data for this year in particular gives every indication of being massaged to within an inch of its life (intra-Shiite violence doesn't count, car bomb fatalities don't count, al-Qaeda attacks against Sunni tribes don't count, the figures change mysteriously from one report to the next, the supposedly lower numbers for August are classified, etc. etc.) We do have figures released by various Iraqi ministries, but we don't have a consistent series of ministry numbers for the past four years, and in any case the numbers for 2007 don't show any decline at all between spring and summer. So that doesn't suggest any surge-related decrease in violence either.
Bottom line: you should be skeptical of any claims about reductions in violence unless they take seasonality into account. So far, though, I haven't seen any credible claims of reduced violence that even mention seasonality, let alone adjust for it. That should tell you something.