text: As a basis for the total lag-to-treatment distribution, the main limitation with the above analysis 
  is that it assumes all individuals coming to treatment do so within ten years. Examining data 
  from earlier cohorts suggests this is inaccurate, as a small proportion of OCUs will continue to 
  use these drugs for a long time, sometimes two decades or more, before seeking treatment, 
  and some never will. However, we cannot use an earlier cohort for the distribution because this 
  is equivalent to using out-of-date data. The average lag-to-treatment is likely to have reduced 
  over time given the expansion of treatment places and the influence of DIP. Using old data will 
  miss this and bias the estimates. Even using the 2005 cohort’s distribution contains the 
  assumption that the time-to-treatment lag has not altered significantly between 2005 and 
  2013/14. So, to try and obtain the most accurate model, we used the figures from the 2005 
  cohort for the first ten years, as above, on the basis that this covers the majority of individuals 
  and for that we want the most up-to-date data possible whilst maintaining a long enough time 
  period. We then index the trend at that point to an older cohort, and use data from that cohort to 
  model the ‘tail’ of the distribution – i.e. those who take longer than ten years to reach 
  treatment.20 The result is a 20-year lag-to-treatment distribution, shown in Table 12 below. 