Actually the way I read the USNWR explanation of that metric, unlike the 6 year graduation rate itself they only use the latest cohort for this measure. So for the current rankings:
They are pretty specific about only using that one year, while in the 6 year graduation rate explanation they specify that they used the 2004-2007 cohorts. So I would presume that for the upcoming rankings they will use the 2008 cohort for this measure and the 2005-2008 cohorts for the other. The 2008 cohort is the one that schools have reported in their 2014-2015 CDS, so that is the most current USNWR will have available to them. So the 83% graduation rate that Tulane reported should be fully realized for this 7.5% factor, while only partially helping the 18% they weight the 6 year rate itself. I still don’t know what they do about Katrina and the N/A report for the 2005 cohort. Maybe for Tulane they will just use 2006-2008. No idea. But at least this year will be the last where it is a direct factor in that sense.