It seems that PolitiFact based its rating on the fact that crime has been on a long downward trend overall for the last 25 years. Volokh writes,
I don't find this a persuasive defense. If the original PolitiFact post had said something like, "The violent crime rate has plummeted in the past 25 years, and while it may have been increasing in the last year and a quarter, that could easily be an anomaly, and our data on that are just preliminary and may not be sound," I would have thought it a sensible criticism of Trump's assertion. We should indeed be cautious about data that are limited to one year, or (as with the 2016 first-quarter data) to a subset of jurisdictions. There is some degree of short-term variation within any long-term trend; data from a year and change aren't really enough to tell whether 1) the long-term violent crime decline has been reversed, or 2) the year was just an anomaly and the decline will continue, or at worst, the violent crime rate will remain flat. For instance, the violent crime rate increased in 2005 and 2006, but those proved to be just small blips in an otherwise substantial decline.
