New opiate and crack-cocaine users: characteristics and trends 19 
 
cent (25,000/150,000).14 It is then possible to model how many of the original population of 
crime-involved OCUs would be likely to test positive in any given year. For example, if there 
were 150,000 crime-involved OCUs in 2008, the chances of one of that group having a first test 
in 2013, providing they remained a crime-involved OCU throughout the period 2004–2013 is 
given by: 
(1 – probability of arrest)^8 = chance of not getting caught between 2004 and 2012 
Multiplied by: 
Probability that they do get arrested in 2013 
This can then be calculated for a range of plausible values for the initial number of OCUs, and 
hence range of arrest rates, to give a range of plausible values for the number of new testers in 
2013 who were actually longer-term users. The results of this modelling suggest that we would 
expect about 2,400–7,000 new positive tests from individuals who are actually longer-term 
OCUs.15 So the fact we only see 4,281 in the real data suggests that genuinely new initiates 
may be a minority within this figure, as many (probably most) will be from the original cohort.  
This is further reinforced by the next set of analyses, which break down the data on new positive 
tests per year by age. Table 8 shows how numbers of unique individuals testing positive for the 
first time break down by year and by age group. The age breakdowns are shown first in 
absolute numbers and in the second table as a proportion of all those with a first test in that 
year.   
Table 8: Unique individuals testing positive for opiates-only or positive-for-both, by age and 
by year of first test.  
Year of first 
test  Age 18-24  Age 25-29  Age 30-34  Age 35-39  Age 40 
over  Total 
2004 3,150 3,319 2,938 1,958 881 12,246 
2005 2,391 2,832 2,548 1,791 977 10,539 
2006 3,635 3,768 3,275 2,491 1,580 14,749 
2007 3,182 3,359 2,869 2,178 1,803 13,391 
2008 2,912 3,197 2,857 2,425 2,238 13,629 
2009 2,711 2,594 2,304 1,998 2,048 11,655 
2010 2,287 2,180 2,105 1,744 2,075 10,391 
2011 1,772 1,519 1,622 1,274 1,726 7,913 
2012 1,136 1,179 1,300 1,030 1,377 6,022 
2013 721 850 938 704 1,068 4,281 
Total  23,897 24,797 22,756 17,593 15,773 104,816 
 
 
14  Note that this rate is, in effect, the rate of arrest-and-testing-positive.  
15  The technical annex contains a section on exactly how this range was estimated.