Some of the numbers, yes, are quite compatible. Some are different because of differences in the way we deploy troops.
For example, in terms of the Americans in Iraq, recently one of their studies found that they had a significantly higher rate of symptomatology suggestive of PTSD six months after their deployment. But that was for 12-month deployments as opposed to six-month deployments.
So there are enough differences that it makes it very misleading and often invalid to compare the results.