text: The majority of those presenting for treatment between 2005 and 2014 started using 
  opiates/crack before 2005 (around four in five). Only 52,829 individuals said they had an 
  opiate/crack initiation date between 2005 and 2014. This suggests an average of just under 
  5,000 new starters per year during this period. But this would be an under-estimate of incidence 
  because it is likely that some of those who began use between 2005 and 2014 would not yet 
  have come to treatment during that period. 
  
  To correct for this, we use two variants of a methodology employed by researchers in Millar et 
  al. (2001) and Hickman et al. (2001). These papers discuss the methodology in detail. 
  
  In brief, the method uses the lag-to-treatment distribution for the sample coupled with the 
  number of new treatment presentations in a given year to estimate OCU incidence in that year. 
  So, when presenting to treatment, all individuals are asked to provide the year in which they first 
  began using their primary drug, which for this analysis was limited to opiates and/or crack- 
  New opiate and crack-cocaine users: characteristics and trends  22 