Sunday, September 13, 2015

A 16% Increase in Murders would be Unprecedented

In response to recent news articles that cited a few cities worth of evidence to show a surge in crime rates, fivethirtyeight compiled a fantastic set of 2015 data for 60 cities to ask how the murder rate has actually changed. They found the rate has risen by 16%, but oddly used that to argue that reports have been exaggerated. But 16% would be an unprecedented increase. See the graph below.
The past thirty years have never seen a 16% increase. This would be an increase of almost 700 murders. (The cities used for this graph are slightly different than the fivethirtyeight sample. This graph is limited to the 56 of the 80 largest cities that reported data every year in the sample.)

Of course, count data isn't ideal for data where population changes. The graph below shows a 16% increase next to historical US murder rates.
Fivethirtyeight's estimate was only for large cities, which likely follow different trends than suburban or rural neighborhoods. So 16% is likely high for the projected US change.

The graph shows a few other interesting points. Although a 16% increase is high, that would only take us to the 2008 murder rate; nowhere near the crime wave of the early 90's. 

I was also surprised to see the high murder rates in the 70's and 80's. I was aware of the high rates in the early 90's, but thought they were unique. The US murder rate more than doubled in the twelve years from 1963 to 1974.

3 comments:

  1. This is a innumerate discussion; the total number of homicides in the US is between 12 K and 15 K for the last few years (depending on where you are getting your numbers from, the BJS or the FBI); the 16% is related to the top 56 coities are 700 increase on a total of 4400; as related to the total number of murders, it will be at the most 5%. We have had drops of 5% and swings of 5% for each year. For a blog concerned with statistics, this is piss poor reading.

    ReplyDelete
    Replies
    1. Sujatha, the blog is clear that the 2015 estimate is based on a 538 estimate and depends on whether it holds true for the larger population. I believe you have also deduced that from, well, reading this blog.

      Delete
    2. and that is the reason that I cannot stand the rise of statistical analysts in finance, baseball or politics; a lack of fundamental understanding of issues, but reliance on year to year swings to predict the future makes them completely unreliable as predictors. 538 and Bill James started this nonsense.

      We can have a bet! at the end of this year, the total number of homicides as reported by BJS UCR or FBI will be no different by +- 2% from last year, or about 200-300 murders. The statistical swings cannot be the basis of policy predictions.

      Delete