text: 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. 
  
  New opiate and crack-cocaine users: characteristics and trends  17 