The extent of the protection conferred by individual anti-burglary devices and selected combinations is discussed in this section. Table 2 presents the information that enables calculating the SPFs. The SPFs for individual devices are shown in Figure 1 and those for combinations in Figures 2 and 3 and Table 3 later in this section. An indication of the statistical significance of the odds ratios is given in both Tables 2 and 3.Footnote 6 Burglary security devices in Table 2 and their configurations in both tables are listed in descending order of sample size in the 2008/2009–2011/2012 CSEW to allow the extent of their use in England and Wales better to be appreciated.
The first three columns of Table 2 present the samples sizes for all households in the sample (hereafter ‘all households’) and for victims of burglary with entry and of attempted burglary. Recall that only security configurations with more than 50 households in the sample are shown. The odds ratios that are given in the last two columns of Table 2 are calculated as follows: The bottom row shows that there were 37 416 households in the sample, of which 2245 experienced burglary and 1356 experienced attempted burglary. The first row of the table shows that 1835 of all 37 416 households (4.9 per cent) reported no security devices. However, 821 of 2245 burgled households (36.6 per cent) reported no security, as did 286 of 1356 (21.1 per cent) of household that experienced attempted burglary. The odds ratios compare the percentage with respect to the two crime types to that for all households. Therefore for households with no security, the odds ratio is 7.46 (calculated as 36.6/4.9) and for attempts it is 4.30 (calculated as 21.1/4.9). The superscript notation shows that these odds ratios are highly statistically significant. In a similar way, the odds ratios of different security configurations can be calculated. However, not all configurations have significant odds ratios. In total, 31 individual devices and configurations show odds ratios that are statistically significant for either type of burglary.
The (multiplicative) difference of odds ratios between any individual device or configuration and no security gives its SPF value. These are shown in the graphs of this section and the second and fourth columns of Table 3 with an indication of their statistical significance. They are calculated as follows: In the second row of figures of Table 2, households with window and door locks have an odds ratio with respect to burglary with entry of 0.59, which is 12.54 (7.46/0.59) times lower than no security. This SPF implies that window and door locks confer 12 times higher protection against burglary with entry than no security. Another way of expressing this is that window and door locks reduce the odds ratio of burglary with entry down to one-twelfth compared with no security. This is considerable compared with previous evidence on car security SPFs that did not exceed 25 (Farrell et al, 2011). It appears, however, modest in relation to other burglary security configurations as will be seen in the following paragraphs. The discussion here uses both interpretations interchangeably.
The SPFs of the individual devices and configurations with at least one statistically significant odds ratios from Table 2, as mentioned, are shown across three graphs, Figures 1, 2, 3. They are listed in descending order of the SPFs against burglary with entry values that is in general more responsive to security than attempted burglary as found in previous research (Van Kesteren et al, 2000). An SPF of 1 implies that the odds ratios of no security and the examined device or configuration are equal and therefore the latter confers no protection. SPFs lower than 1 imply that the respective device or configuration is counter-productive: the odds of burglary are actually lower without any security than the device examined. Therefore the following discussion and related graphs of SPFs use 1 (rather than 0) as baseline (the value of the y-axis at which the x-axis intersects) which best reflects the protection conferred by each burglary device configuration.
It remains relatively common to use a single type of security device, as shown by the sample sizes in Table 2. As expected, the SPFs for individual devices tend to be lower than those for combinations in Table 3. For visual clarity the SPFs of individual burglary security devices across crime type are also given in Figure 1. When devices are used individually, window locks have the highest SPF followed by indoor sensor lights, but neither has statistically significant odds ratios and for this reason their respective SPFs are shown in solid white bars. This suggests we cannot conclude with confidence that they confer greater than no security. Given that the Home Office classifies window locks together with door locks as basic security, the fact that on their own they do not confer statistically significant protection comes as a surprise. The reasons for this remain uncertain and further research might clarify that issue. Perhaps window locks fall quickly into disuse if they are add-on rather than built-in, as may be more likely in older households with less security generally. Perhaps integrated (built-in) window locks work automatically when a window is closed whereas add-on locks typically require the fitting of a nut onto a bolt, producing significant variation in usage between window locks (perhaps particular in summer months when windows are opened more frequently). Future analysis on modus operandi in burglary incidents recorded by the CSEW will shed some light on this. In addition, window locks are an essential built-in feature of double-glazed windows and therefore their widespread availability (see Table 1) has possibly more to do with heat insulation and less with burglary security. Indeed Table 2 shows that they are available to burgled households at the same rate as in the general population. Similarly, indoor lights on sensor and window bars have statistically non-significant odds ratios for burglary with entry and attempted burglary (solid white bars in Figure 1), respectively. This might be partially because of the low number of households using indoor lights (47 or 0.13 per cent) or window bars (21 or 0.06 per cent) as the only security feature of their homes (see Table 2).
It is evident from Figure 1 that individual devices are much more effective against burglary with entry than attempts which is an anticipated and welcome result. Of the statistically significant results, external lights or door locks confer the highest protection (SPFs of 3 and 2.8, respectively, for burglary with entry) among all individual devices, that is, assuming the presence of no other. Previous research, some based on offender interviews, has indeed identified high visibility by overlooking occupied properties or passers-by as a main burglary deterrent (Coupe and Blake, 2006; Coupe and Hahn, 2010). Door locks increase the effort and time needed for breaking into a property and thus the risk of the potential burglar being interrupted by passers-by or neighbours (Chenery and Pease, 2013). Security chains are roughly twice better than no security while CCTV, dummy alarm and window bars confer some protection against burglary with entry but none for attempts.
Burglar alarms and dummy alarms appear to increase risk of attempted burglary. The SPFs of dummy alarm for attempts and burglar alarm for any type of burglary are less than 1 implying a counter-productive effect: having just a burglar alarm and no other form of security confers less protection than no security at all. This is counter-intuitive but not without some plausible explanations. It is conceivable that a burglar alarm in a house with no other security features may flag the existence of valuables relative to other ‘unsecured’ properties. In addition, as with the window locks discussed earlier it requires some action on the part of the householder to become functional against burglary. Its simple availability unless set does not by definition confer any active protection, but even worse it may give a false sense of protection that makes such households ‘careless’. Another plausible explanation is that burglar alarms were fitted to homes that had been burgled before the CSEW reference period and therefore the respective households are repeat burglary victims, although the data cannot confirm this. There is ample research evidence that victims have a higher risk of a subsequent (repeat) burglary and that policies that prevent repeat burglary reduce the overall burglary rates (Pease, 1998; Grove et al, 2012). Therefore the repeat victimisation risk may counterbalance the potential protection conferred by the burglar alarm. It is also possible that respondents to the CSEW may misinterpret false alarms as attempted burglary. However, dummy alarms also do not offer any active protection and so their protective effect against burglary with entry is hard to explain. One possibility is that burglars believe a silent alarm, and perhaps a personnel response, has been triggered if there is no audible alarm. These results suggest that a range of issues exists for future research including the role of modus operandi of burglars and the attributes of households and residential areas.
Device pairs and triples
Generally speaking, protection increases with the number of devices, as evidenced by the different maximum ordinal scale values of Figures 1, 2, 3. The SPFs of pairs and triplets of security devices are given in Figure 2. As mentioned, the exact SPF values can be found in Table 3 (second and fourth column). These results suggest that triples are more effective than pairs. The exception is the pairing of external lights and window locks (EW) which, for burglary, is more effective than the IWD triplet (indoor lights, window locks and door locks). Examining, first, pairs of security devices burglar alarm and door locks (BD) confer no protection for attempted burglary but nearly half the risk of burglary with entry compared with no security. By contrast, window and door locks (WD), the Home Office basic security configuration, reduce the odds ratios of burglary with entry or attempts to roughly 1/13 and 1/8, respectively. This is a considerable effect which, compared with the SPFs of pairs of car crime prevention devices calculated by Farrell et al (2011) , suggests that burglary is much more responsive to target hardening than car crime. The remaining three pairs that incorporate window locks, that is, together with external lights (EW), security chains (WS) or burglar alarm (WB) confer statistically significant protection only against burglary with entry with respective SPFs at roughly 18, 13 and 5. Owing perhaps to the great effectiveness of external lights, their combination with window locks confers the highest protection against completed burglaries, among all pairs of devices with significant odds ratios, including WD, the Home Office’s basic security. Finally external lights and security chains (ES) have a small protective effect against attempts.
Considering now the configurations of three security devices, that is, triplets, in Figure 2, it is clear that they generally confer double the protection provided by pairs. As perhaps anticipated, the highest (roughly 34 and 14 SPFs for completed and attempted burglary, respectively) is offered by external lights, window and door locks (EWD). Among the remaining triplet combinations the addition to window and door locks of security chains (WSD), CCTV (CWD) or indoor lights (IWD) confers roughly 32, 24 and 17 times higher protection against competed burglary than no security. Their protection factor against attempts is 8, non-significant and 25, respectively. The magnitude of the protection against burglary conferred by triples is considerably greater than that against attempts. For the three triplets conferring greatest protection against burglary, they confer more than double the protection against attempts. However, the combination of indoor lights, window and door locks (IWD) is one out of two (the second such configuration discussed in the next paragraph) that confers more protection against attempts than burglary with entry. As will be seen next, EWD and WSD confer more protection than most combinations of four or more devices.
Device quadruples and greater
Figure 3 presents the SPFs of combinations of four or more security devices with significant odds ratios. The SPFs with regard to burglary with entry are discussed in this and the next two paragraphs. The most striking feature is the extraordinary SPF of 139 against burglary for the combination of all seven security devices examined here except CCTV (EIWSBD). These devices in combination were reported by 931 households (see Table 2) in the 2008/2009–2011/2012 CSEW and the effect is statistically reliable albeit a clear outlier that has not been replicated in preliminary (not shown here) analyses of previous CSEW sweeps. The respective bar has been truncated in Figure 3 to allow the effects of the remaining configurations better to be appreciated. The second most surprising finding shown in Figure 3 (and in comparison with Figure 2) is that burglary protection does not consistently increase with the number of devices that make up each configuration.
The second and third highest SPFs against burglary (after the above-mentioned outlier effect) are, perhaps surprisingly, delivered by the combination of only four security devices out of the seven examined. One of these high-impact four-way combinations includes CCTV, window and door locks and security chains (CWSD). It confers 52 times more protection against burglary compared with no security. The other refers to lights (external and indoor) and locks (for windows and doors) (EIWD) that confers similar protection by a factor of 49. Window and door locks, security chains and external lights (EWSD) confer protection by a factor of 31.5, whereas the remaining four devices configurations, that is, IWSD, WSBD, IWBD and EWBD, reduce burglary odds ratios by a factor of between 23 and 16.5 in the above order.
Looking at combinations of five devices the most effective protection against burglary is conferred by external lights, window and door locks, burglar alarm and either CCTV (CEWBD) or security chains (EWSBD) – by a factor of about 32. Similar protection is found for the two quintuplets of locks and lights with either burglar alarm or security chains, that is, EIWBD and EIWSD, with SPFs of about 30 and 28, while locks, security chains, burglar alarm and indoor lights (IWSBD) reduce burglary with entry odds ratios to 1/23 of that of no security. Finally, households reporting the combination of all devices except security chains (CEIWBD) are protected against burglaries 34 times than those having no security.
Turning now our attention to attempts in Figure 3, the highest protection is conferred by EIWSD which is the second burglary devices configuration to affect attempts more than burglary with entry. EIWD and EWSD are the third and fourth most protective configuration against attempts while the second and fifth place are held by triplets: IWD, mentioned earlier for its higher effectiveness against attempted than completed burglary, and EWD.
The more the merrier?
One question one may ask is whether the SPFs of burglary security combinations are a straightforward extrapolation of the SPFs of the individual devices that make up each configuration. The answer is that they are not. For example, the impact of car security configurations against theft of car is greater than the expected from the individual contributions of the devices that make up each combination (Farrell et al, 2011). The difference between expected and observed SPFs gives the net interaction effect (NIE) which is calculated as follows: As seen earlier in Figure 1 the SPF for window locks, W, is 6.58 (albeit non-statistically significant) and that for door locks, D, is 2.79. The sum of the two individual impact factors is the expected protection from their combination, WD, and equals 9.37. In Figure 2 and the first row in Table 3, however, the WD configuration has an SPF of 12.54. This exceeds the expected impact by 3.17 which is the NIE of this particular combination. The third and fifth columns of Table 3 give the NIEs of security configurations against burglary with entry and attempts, respectively. Non-surprisingly the greatest NIE (120) refers to the combination of all security devices except CCTV (EIWSBD), which had extremely high impact against burglary with entry. The next four greatest NIEs for the same crime type refer to the following configurations in descending order: CWSD (38.9), EIWD (33.2), EWD (22) and WSD (20.9). The five configurations with greatest NIE with respect to attempts are EIWSD (22), IWD (18.2), EIWD (13), CWSD (10.5) and EWSD (8.9).
Two points are worth mentioning here. First, the number of devices is not the main driver in burglary prevention. It is rather the effectiveness of a particular combination. For instance, the second best SPF and NIE against attempts is provided by the combination of only three devices: IWD. Second, the magnitude of the NIE roughly reflects the SPF value for each security configuration but not always. For example, WSD has a higher NIE but lower SPF against burglary with entry than, say, CEIWBD (15.6), EWSBD (17.1) or CEWBD (17.6).
Protection against burglary and attempts does not consistently increase with the number of devices that make up each configuration. This is evidenced in Table 4 that shows the mean SPF protection across the different number of burglary devices per combination (in descending order with respect to burglary with entry). Protection increases greatly from two to three devices against both burglary and attempts. The mean level of protection conferred against burglary by three devices is, however, almost as high as that of four or five devices. Against burglary, the SPF means of four and five devices are almost identical. The protection conferred against burglary is always greater than that against attempts, irrespective of the number of devices, as shown in the final column as the ratio of the SPF of burglary to that of attempts. On average, protection conferred against burglary is three times that of attempts, although this is skewed by the SPF of six devices against burglary (Table 4, last row). If six-device combinations are excluded, the mean SPF conferred against burglary is double that against attempts. Consequently, as might be expected, it is not simply a case of ‘the more the merrier’, as the types of devices that are combined has an effect. More precisely gauging the marginal effect of devices added to particular combinations may be an area for further research.