New opiate and crack-cocaine users: characteristics and trends 17 
 
Despite this, there are still a few things these data can tell us about the numbers and trends in 
new OCUs. Firstly, despite the downward bias outlined above, it is clear that there are not 
enough new crime-involved OCUs entering the population to cause this trend to increase or 
even flatten. Secondly, it is assumed that the probability of arrest stays roughly constant through 
the period, the trend in the number of new positive testers, who are actually captured from the 
original cohort, will gradually flatten, whereas the decline in new OCUs in the DIP data is almost 
linear, suggesting that numbers entering the population are also falling. But most important is 
that the absolute number of new positive testers in 2013 is only 4,281. Given that this figure is 
likely to include some individuals who are actually long-standing OCUs, but who have evaded 
arrest to that point, then the number of genuinely new arrested OCUs in 2013 must be lower 
than 4,281, possibly markedly lower. 
To investigate this further, the next two tables break down annual totals for all tests and all 
unique individuals, by the year of first test. 
Table 6: Number of positive opiates-only or positive-for-both. tests, by year of first positive 
test.  
  Number of tests per year (positive opiate/opiate + cocaine) 
Year of first 
test 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 Adjusted 2013 
2004 17,174 5,604 7,091 6,784 6,509 5,292 4,863 3,341 2,629 1,800  1,964  
2005   13,553 6,066 5,110 4,941 3,983 3,549 2,323 1,947 1,383  1,509  
2006     20,656 7,784 6,152 5,139 4,629 3,257 2,649 1,806  1,970  
2007       17,613 5,747 4,309 3,855 2,619 2,119 1,555  1,696  
2008         17,883 4,970 4,026 2,626 2,180 1,562  1,704  
2009           14,683 4,054 2,383 1,824 1,318  1,438  
2010             13,075 2,332 1,638 1,154  1,259  
2011               9,595 1,714 1,013  1,105  
2012                 7,265 1,359  1,483  
2013                   5,523  6,025  
Total 17,174 19,157 33,813 37,291 41,232 38,376 38,051 28,476 23,965 18,473  20,152  
 
  
                                                                                                                                                                                                                 
involved opiate/crack users will quit (or die) before being arrested and tested; 3) DIP’s geographical coverage is not 100 per 
cent; 4) Some may evade arrest through the entire series; and 5) Evidence suggests OCUs cycle in and out of periods of 
regular use and offending rather than offend at a high rate continuously. But clearly the gradual capture of the pre-existing 
population creates a big enough bias such that we cannot read the figures for new positive testers simply as an incidence 
trend for crime-involved opiate/crack users.