text: A full discussion of DIP’s geographical coverage over time is contained in the Appendix, which 
  also shows how the available data break down by local authority area. While there is some 
  variation in the number of local authorities returning DIP data, particularly post-2006, areas with 
  higher test volumes are well covered throughout the period. So, while any trend should be 
  treated with care, more confidence can be taken in the analysis of the year of birth and age 
  characteristics. 