Tag Archives: homicide rates

Year-to-date comparisons and why we should stop doing them

Year-to-date comparisons are common in both policing and the media. They involve comparing the cumulative crime count for the current year up to a certain date and comparing to the same point in the preceding year. For a Philadelphia example from April of this year, NBC reported that homicides were up 20 percent in 2017 compared to 2016. You can also find these types of comparison in the Compstat meetings of many police departments.

To gauge how reliable these mid-year estimates of doom-and-gloom are, I downloaded nine years (2007-2015) of monthly homicide counts from the Philadelphia Police Department. These are all open data available here. I calculated the overall year change as well as the cumulative change monthly from year to year. In the table below you can see a row of annual totals in grey near the bottom, below which is the target prediction as a percentage of the previous year (white text, blue background). For example, the 332 homicides in 2008 were 14.7% lower than the previous year, expressed in 2007 terms.

Let’s determine that we can tolerate our prediction to be within 5 percent plus or minus the eventual difference between this year and the preceding year. That stipulates a fairly generous 10% range as indicated by the Low and High rows in blue.

Each month you can see the percentage difference between the indicated year-to-date at the end of the month, and the calendar year-to-date (YTD) for the same period in the previous year. So for example, at the end of January 2008 we had 21.9% fewer homicides than at the end of January 2007. By the time we get to December, we obviously have all the homicides for the year, so the December percentage change exactly matches the target percentage difference.

Cells highlighted with a green background have a difference on the previous year that is within our +/- 5 percent tolerance. By the end of each January, we only had one year (2012) with a percentage difference that was within 5 percent of how the city ended the year. The 57% increase in January 2011 was considerably different that the eventual 6% increase over 2010 at the end of December. When Philadelphia Magazine dramatically posted “Philly’s Murder Rate is Skyrocketing Again in 2014” on January 14th of that year, the month did indeed end up nearly 37 percent over 2013. But by year’s end, the city had recorded just one homicide more than the preceding year – a less dramatic increase of 0.4%.

In fact, if we seek out a month where the difference is within our 10% range and later months will remain consistently accurate through to the end of the year, then we have to wait until the months shown with a border. 2009 performed well, however while 2010 was fairly accurate throughout the summer, the cumulative totals in September and October were more than 5% higher than the previous year when the year ended only 0.3% higher.

To use calendar YTD comparisons with any confidence, we have to wait until the end of October before we can be more than 50% confident that the year-to-date is indicative of how we will enter the New Year. And even then we still have to be cautious. There was a chance at the end of November 2010 that we would end the year with fewer homicides, though the eventual count crept into increase territory.

The bottom line is that with crimes such as homicide, we need not necessarily worry about crime panics at the beginning of the year. This isn’t to say we should ever get complacent and of course every homicide is one too many; however the likely trend will only become clear by the autumn.

Alternatives exist. Moving averages seem to work okay, but another alternative I like is to compare full (annual) YTDs to the prior annual (i.e. full 12 month) YTD. So instead of (for example) comparing January-April 2010 to January to April 2009, you could compare the 12-month change May 2009-April 2010 against the May 2008-April 2009 total. I’ve done that in the red graph below. The first available point is December 2008 and as we know from the previous table, the preceding 12 months had outperformed the annual year 2007 by 14.7%. But then each subsequent month measures not just the calendar YTD but the 12-month YTD.

The result is a graph that shows the trend changing over time from negative (good) territory to positive (bad for homicides because it show an increase). Not only do you get a more realistic comparison that is useful throughout the year, you can see changing trend. Anything below the horizontal axis is good news – you are doing well. Above it means that your recent 12 months (measured at any point) was worse than the preceding 12 months.

You can have overlapping comparison periods. The graph in blue below compares 24 months of accumulated counts with the 24 month totals for the previous year. For example, the first point available is December 2009. This -11.7% value represents the change in total homicides from the 24 months January 2008 to December 2009 and compares it to the 24 month total from a year previous to this (January 2007 to December 2008). For comparison purposes, I have retained the same vertical scale but note the change in horizontal axis.

You can see there is more smoothing, but the general trend over time is still visible. Lots of variations available and you might want to play with different options for your crime type and crime volume.

Harm-focused policing

On 28th January, 2015 I gave the Police Foundation‘s Ideas in American Policing lecture on the topic of harm-focused policing. This brief blog provides some background details to the talk. Please note that for a number of reasons (including photograph copyright) I am not distributing copies of the PowerPoint slides.

Harm-focused policing weighs the social harms of criminality and disorder with data from beyond crime and antisocial behavior, in order to focus police priorities and resources in furtherance of both crime and harm reduction.

Example information and data sources could include drug overdose information that could help triage drug markets for interdiction, traffic fatality data to guide police patrol responses, and community impact assessments to prioritize violent street gangs. For a summary of the core of the presentation and a grey-scale version of some of the graphics, please see:

Ratcliffe, J. H. (2015). Towards an index for harm-focused policing. Policing: A Journal of Policy and Practice, 9(2), 164-182.

You can visit the journal site and access the paper here (or here) and watch my annotated video of the lecture below (you might want to make it full screen so you can read the slides).

During the presentation, I had a couple of quotes. Here are the quotes and their sources.

“to establish priorities for strategic criminal intelligence gathering and subsequent analysis based on notions of the social harm caused by different sorts of criminal activity”. The source for this is page 262 of Ratcliffe, J. H. & Sheptycki, J. (2009) Setting the strategic agenda. In J. H. Ratcliffe (ed.) Strategic Thinking in Criminal Intelligence (2nd edition) Sydney: Federation Press.

“Weighting crimes on the basis of sentencing guidelines can be justified on good democratic grounds as reflecting the will of the people. …… it remains far closer to the will of the people than any theoretical or even empirical system of weighting that academics might develop.” The source for this is Sherman, L. W. (2013). Targeting, testing and tracking police services: The rise of evidence-based policing, 1975-2025. In M. Tonry (Ed.), Crime and Justice in America, 1975-2025. Chicago: University of Chicago Press. Page 47.

 

Why we shouldn’t fixate on homicide numbers

There are some certainties in life. Death, taxes, the Eagles snatching defeat from the jaws of victory. And the annual January media fixation with homicide rates as the barometer of everything from a city’s moral compass to the effectiveness of the police chief.

I spent a couple of days speaking to various reporters about the homicide numbers in Philadelphia, and how they were significantly down on a few years ago, but had remained largely unchanged since last year. ‘What could we gather from this?’ ‘What were the implications?’ ‘Were police department strategies starting to falter?’ ‘What does it mean for the mayor and police commissioner?’

Taking more time than I really had, given I am trying to update ‘Intelligence-Led Policing’ for a second edition, I tried to explain that the homicide figures are a really bad choice of metric. For just about anything. For example, a not insubstantial number of homicides occur between people who know each other, and often take place indoors. How are the police department supposed to anticipate and prevent those homicides? Even if they develop a ‘Minority Report’ predictive capacity, we have a reactive legal and criminal justice system: it isn’t keen on letting the police just wander into your house and lock you up for pondering murder. And for the homicides that take place on the street? Sitting in on numerous Philadelphia Police Department crime briefings and listening to the homicide reports, it is clear that many are the result of minor disputes that flared up with little-to-no warning or are the result of disputes between participants in gangs or drug organizations who conceal their business and would never seek the intervention of the police.

The difference between a homicide and an aggravated assault is also largely outside of police control. Could be the shooter has lousy aim or is firing gangster style, there is a delay in getting the victim to the hospital, or simply medical mismanagement. Once a person decides to shoot someone else, they are easily able to in the US because we allow them the opportunities to do so. Our legislators seem unwilling to help the police with this, so again, little chance for police influence here.

I examined a summary of every incident recorded by the Philadelphia Police for the last available full year (2013) to estimate how much police patrol energy is expended on responding to homicide incidents. In Philadelphia, the city receives millions of calls for service, and from these – as well as police-generated activity – an INCT database is created. This database contains every incident where a police officer was required to act, and ranges from dog bites and graffiti to shootings and homicides, and from assistance to city agencies and delivering messages, to removing debris from the interstate or arresting a drunk driver. In 2013 there were in excess of 1.65 million incidents. What percentage of these related to homicide? 0.021%. Less than one quarter of one tenth of one percent.

I explained to the reporters that aggravated assaults and robberies were also down, and due to their greater number generally, this was a much better way to indicate the crime health of the city. They said they got it, but their hands were tied: “the public interest is in homicides”.  So we still got story after story about the homicide rate. Not a major grumble: reporters have to make a call and write what they think is the story. But I wonder if the fascination with homicides is really driven by public demand, or by the media? I can’t believe there was massive public outcry that drove the claim that “Philly’s Murder Rate Is Skyrocketing Again in 2014”… especially only two weeks into the year (you had to go back and checked the post date didn’t you?).

Traditionally, homicides have been used because they are easily comparable between cities, because police departments have recorded other incidents in different ways, or because in the past (sometimes not so distant) the police have distorted the crime figures. But homicides comprise so little of the work of a police agency, and the chances of most people being a victim of homicide are so low, that they tell us little about the experienced crime rate or the quality of life for city residents.

We need to start moving to more holistic measures if oversight and strategy are to be more data driven and evidence based. Harm-focused policing that examines and weighs all incidents, and includes other harms to communities, such as traffic accidents or the potentially deleterious impact of unrestrained pedestrian investigations, is increasingly possible with the big data sets that public agencies generate. We need to evolve beyond our fixation with homicide if we are to move the discussion about safety and harm forward.

But in the meantime, Philadelphia, be glad that shootings and robberies are also down.

(This post was updated shortly after posting to correct the homicide incident rate)