text: i)**Stock and flows:**Simply knowing the stock of OCUs tells us nothing about the flows in 
  and out – i.e. if the stock were constant each year that could mean that no one starts 
  using these drugs and no one quits or it could mean all existing users quit but that they 
  are wholly replaced by new users, or any similar scenario in between. Clearly the policy 
  response would need to be quite different for each of these cases, so knowing the true 
  situation is important. 
  
  ii)**Early-warning system:**Research by the Home Office and others has shown that there 
  is generally a lag between the start of a heroin/crack epidemic and the point at which it 
  becomes visible on administrative datasets. Closing this gap is important for policy, and 
  part of the reason for its existence is the lack of incidence estimates. Evidence also 
  suggests epidemics spread from area to area, so it is important to monitor local as well 
  as national trends. 
  
  iii)**The social harm that can arise:**Though research suggests that not all OCUs resort to 
  
  acquisitive crime to help finance their drug use, numerous studies show that a 
  proportion consistently do and these individuals can be extremely prolific offenders 
  (Morgan, 2014). One study by Frontier Economics estimated that the average lifetime 
  cost to society of an injecting drug user was £445,000 from crime alone. Hence 
  analysing and identifying new OCUs is a policy priority (Frontier Economics, 2010). 