The spatial behavior of residential burglars Manuel J.J. ... - TU Delft

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426 Manuel J.J. López Figure 200: Range of operation and buffer zone at least five burglary offences that were committed between 1998 and 2004. When we look at the sample, we see that none of the offenders were specialists in the sense that they did not limit their criminal acts to residential burglary. They were all male and between the ages of 21 and 43. The mean age was 31.8 (S.D. = 0.6). Seven offenders were registered by the police as habitual offenders, one was an alcoholic and one was a drug addict. The average number of burglary offences by one offender was 11, with a minimum of 5 and a maximum of 19 (S.D. = 3.9). Half of the offenders (10) were born in Kennemerland and the other half were born elsewhere in The Netherlands (8) or abroad (2). A minimum of five maps was made for each offender. The first map showed the different home addresses of the offender during his lifetime. A distinction was made between the offender’s home address before, during and after the period of offending. The second map marked the locations of the burglary offences and the remaining maps gave different representations of the relationship between the offender’s home and burglary locations. 3. Range of operation The first step in the study was testing the hypothesis about the range of operation. When we know both the location of the offender’s home base and the location of his offenses, we can easily determine the range of operation by measuring the longest straightline distance between the home location and offence locations. The so-called buffer zone can be determined in the same fashion, by measuring the shortest straight-line distance between the home location and offence locations. Both the range of operation and the buffer zone can be represented on a map, by drawing both distances around the home location with a pair of compasses (Figure 200). When we reverse this procedure and draw the longest straight-line distance not around the location of the offender’s home but around the locations of his offences, we see that the offender’s home base is in the area of the biggest overlap (Figure 201). If we both know the location of the offences and the distance of the range of operation, we can be 100% accurate in defining the area in which we can expect the offender’s
The spatial behavior of residential burglars 427 Figure 201: Defining the search area home base. However, criminal investigation is not always successful and most crimes are not solved. 4 The police may have indications that certain offences are committed by the same offender (e.g. through DNA or other clues), but may not know the identity of the offender and his range of operation. For such cases we have a couple of techniques that can help. Canter and Larkin (1993) measure the maximum distance of offence locations within a crime series as the range of operation. This method might seem arbitrary, but it is based on the empirical finding that the maximum distance of offence locations usually closely matches the longest straight-line distance between the home location and offence location (the range of operation). Van Koppen et al. (2000) present another solution. They measure all distances between the home and offence locations for every known offender, offence type, and geographic area. Then, the median of these distances is calculated. The resulting value is the range of operation of all offenders within that geographic area. Both methods have their pros and cons. The method of Canter and Larkin is offender specific and therefore does justice to the variations of mobility of the different individual offenders. This method also often succeeds in identifying the correct search area (the area in which the home location of the offender is suspected), but in some cases this area is so large that it has no additional value for the criminal investigation. The method of Van Koppen et al. is not offender specific as it chooses one range of operation for all offenders of a certain offense type in a given geographic area. The advantage of this approach is that the range of operation that will be used for criminal investigation is never too big. It always produces search areas that are small; thus, they offer additional value for the criminal investigation. However, the method also has two major disadvantages. The first disadvantage is the fact that it is based on the incorrect assumption that the radius in which he commits 50 percent of his offences can identify the offender’s range of operation. 5 There is also the disadvantage that the method does insufficient justice to the fact that different offenders most often have different ranges of operation. Given the disadvantages of both methods, this study developed an alternative approach. This third method combines the two earlier methods. First, it identifies the median of the ranges of operation of all known offenders of a certain offence type in a given geographic area. Secondly, the method 4 In 2000 the national clearance rate for residential burglary was 7.2% (López and Reijenga 2001). The clearance rate in Kennemerland was a little higher as 9.8% of residential burglaries had resulted in the conviction of one or more suspects. 5 The idea to take the median rather than the average is a good one, but should actually be applied on the longest distance between the offender’s home and offence locations (the real range of operation).
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