Every time Mayor Bill de Blasio says or does something involving the NYPD, he seems to trip over himself.

First, there was his ill-considered telephone call to a deputy chief to check on the arrest status of campaign supporter Bishop Orlando Findlayter. Then, after a speech detailing his citywide initiative to curb traffic fatalities, his motorcade was caught blowing through a couple of stop signs. Last week, the mayor pointed to statistics that showed that since taking office 10 weeks ago, the number of murders had fallen 21 percent and shootings had dropped 14 percent compared with the same period a year ago.

The reason for the declines, he said: the de Blasio approach to policing.

However, you can't conclude anything with certainty on a scant 10 weeks of data.

"The usual rule in social science is you have at least one year's data to draw any conclusions," says the NYPD's unofficial official historian, Thomas Reppetto. "You can't just go from month to month because things will change and the good narrative will be used against you as a bad narrative."

Moreover, those 10 weeks occurred during one of the city's most bitter winters. That's when crime drops. Criminals, like everyone else, don't like to go out in the cold.