New opiate and crack-cocaine users: characteristics and trends 9 
 
cocaine users. In addition, the sharp decline in total DIP tests in 2013 may be due in part to the 
fact that DIP ceased to be a nationally funded programme in April 2013.  
These data do show, however, that from 2006 onwards, between a third and half of all 
acquisitive crime arrests involved a drug test and between 15 per cent and 35 per cent of those 
tests (depending on the year) resulted in a positive result for opiates-only or for both opiates and 
cocaine (hereafter labelled `positive-for-both’).   
The reason for highlighting only the opiates-only and the `positive-for-both’ test results is that 
the primary group of interest in this report are opiate and crack-cocaine users. To capture this 
group, cocaine-only tests must be excluded because DIP tests cannot distinguish between 
powder- and crack-cocaine, so a cocaine-only positive test could indicate either. Previous 
evidence has demonstrated that while there is much overlap between heroin and crack-cocaine 
cohorts (i.e. many of those who use heroin also use crack-cocaine), powder-cocaine users have 
a quite different profile and are far less likely to be involved with acquisitive crime. Excluding the 
cocaine-only tests means we can be guaranteed not to capture any powder-cocaine users (who 
are not also using opiates or crack), but it also means we may miss some crack-cocaine-only 
users, hence the figures may under-estimate the true population of OCUs slightly.  
The fifth row in Table 1 shows that the total number of opiate and opiate/cocaine tests over the 
period was 364,537. Table 2 shows descriptive statistics for the individuals providing these tests 
(noting that the same individual may be included several times if they gave multiple positive 
tests).   
Table 2: Descriptive statistics on all positive opiate-only/positive-for-both tests. 
Opiate/opiate+cocaine positive tests in England 2004–2013 (all positive tests including repeats 
by the same individual) 
Age Year of birth 
Number of tests  364,537 Number of tests 364,537 
Mean 32 Mean 1977 
Median 31 Median 1977 
Mode 28 Mode 1979 
Minimum 18 Minimum 1960 
Maximum 53 Maximum 1995 
 
The mean age at test is 32 and the mean year of birth is 1977, implying that most of these 
individuals were in their mid-to-late teens during the crime peak of the mid-1990s.9 Given 
evidence suggesting that the average age of initiation for opiate/crack use is around 18–20 
(Millar et al., 2001), this age profile would tentatively suggest that OCU incidence also peaked in 
the 1990s and that this created a large cohort of users who would be approaching 40 today.  
The minimum and maximum years of birth are fixed by construction, because anyone born 
 
9  Note that the dataset counts tests, not unique individuals, so the same person can appear more than once.