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Friday, 4 February 2011

Mapping and Classifying variations Crime and Disorder in forces in England and Wales in December 2010 or variations in recording practices?

In my last post I said that there are fundamental problems with the latest UK crime mapping site. I do not like to be negative about things that people have worked very hard to produce so I created the map above to illustrate  the problems and benefits of this initiative.

Firstly how did I create the map?
  1. I downloaded the neighbourhood data for all the forces (unfortunately the Sussex spreadsheet was empty) and created a force total of the five categories of crime and one category of anti-social behaviour for December 2010.
  2. I attempted to compensate for the different sizes of the forces by dividing the counts in the six categories by the latest published total police officer and staff numbers for each force. This has its problems (and benefits which I will argue elsewhere) but it is better in my opinion than dividing by resident population.
  3. I then wanted to cluster the data giving equal weight to each category (see my previous posts regarding clustering) so I standardized all the data by transforming them into Z scores. This shows the standard deviation either side of the mean creating positive and negative figures.
  4. I then decided the best probable number of clusters by using the Ward's Hierarchical method in SPSS 17 and choose 6. I then used the K means clustering method with the six clusters.
  5. The K means method allows a better understanding of what factors has influenced the clustering, allowing a classification of the clusters to be reliably undertaken. This classification is shown under the map above with a table showing the scores for the cluster centres.
  6. Now this is where I need help. I cannot find a shapefile for police forces in England and Wales. Edina has a shapefile for police force basic command units for England and a seperate one for Wales. I had to do a bit of editing to the .dbf file to show my classification at force level using ArcGIS.
Some very interesting groupings have resulted and vast differences in the recorded level of crimes and ASB has become apparant.

But what are the data I am classifying and mapping here?

To be honest you I do not know, I could find no documentation to explain. Now I am assuming that the crimes are police recorded crime in Home Office Crime Types but it would not surprise me to find out that there have been problems in ensuring that every force is submitting exactly the same types of crime. The chances of that being uniformly the same is a lot higher than the Anti-Social Behaviour figures, which I am assuming are from the police incident recording database and would not surprise me if they differ significantly in content from force to force.

So that is the number one fundamental problem - what is being counted, how is it counted and is it the same through out the country?


  1. EDINA does have a shape file for police force boundaries, within the UK BORDERS section, called 'English Police Force Areas, 1991'. You have to change the 'Dates' drop-down menu to '1990 to 1999' to see it.

    I've had a quick look at it and it appears to actually show the situation post-1999, when the Metropolitan Police District (MPD) became coterminous with Greater London excluding the City. I don't think there have been any other changes in force area since the Local Government Act 1972, so it should be accurate.

    The only slight anomaly is in the boundary between Barnet (in London) and Hertsmere (in Hertfordshire), which deviates slightly from the local authority boundaries. I think this is most likely to be an error, since it shows about 95% of Hertsmere to be in the Hertfordshire Constabulary area, whereas prior to 1999 the whole district was in the MPD, if I recall correctly?

  2. The 2001 BCU shape file on EDINA seems to have the same problem - it shows the border between Herts and the MPS in exactly the same way as the 1991 police force area boundaries shape file does.

    The problem is that the police shape files show Barnet borough as including a triangle of land, two sides of which are described by the A1 and A1081 roads as they go south from Junction 23 of the M25. According to the EDINA shape file for local authorities (and the OS Meridian 2 map on Digimap Roam) this area is in Hertsmere, not Barnet.