33% reduction in public order prisoners in 2002

Did the aftermath of 9/11 attacks result in fewer people being imprisoned for public order offenses?

I happened across interesting graph on a page on the US Department of Justice Web Site. This is of course interesting for many reasons that are already well known, such as the huge increase in prison population and the particular large percentage increase in people imprisoned for drug offenses in the late 80's and early 90's.

But there was one unexpected thing I noticed: a precipitous decline from 2001 to 2002 in the number of prisoners imprisoned for public order offenses, which include "weapons, drunk driving, escape/flight to avoid prosecution, court offenses, obstruction, commercialized vice, morals and decency charges, liquor law violations". This is more apparent in the graph showing the year-over-year change:

So what happened between 2001 and 2002?

My first thought was that it was something to do with the 9/11 attacks and their aftermath. Perhaps the surge of patriotism caused people to be better behaved, resulting in fewer public order offenses. Or perhaps the increased military recruiting had swept up people who committed, or would have committed, public order offenses.

But then I showed the first graph to my wife, and she immediately proposed that the first thing to be suspicious of was whether there had been a change in the way offenses were classified. She sees that kind of thing all the time in the data she works with.

Eventually I found a Q&A; on Ask MetaFilter discussing this issue. Several of the postings there also proposed that the classification has changed, and indeed the third graph below shows that the count of people in prison for other offenses had a significant increase in that same year.

So, it seems likely that this is not such an interesting phenomena after all, just a statistical artifact.