For the ONC process to produce unbiased estimates of the population it is necessary for the census and Census Coverage Survey to be as independent of each other as possible. As described above, practical arrangements were put in place to achieve this with census and CCS operations being kept entirely separate on the ground. If the two attempts at enumerating the same population are independent, it is possible to not only estimate those missed by either the census or CCS but to also estimate those missed by both - the dual system approach.
Through this approach, independence of process was achieved. However, there is an additional component of dependence which needs to be taken into account. This is dependence caused by the fact that those people who are difficult to count in a census are also difficult to count in a post-enumeration survey such as the CCS. This was expected and a methodology was developed to identify those areas where dependency was marked and to adjust for that dependence. This added an additional 230,000 to the ONC population estimates for England and Wales as a whole.
This process ensures that the total estimates include allowance for those people more likely to have been missed by both the census and CCS. (A Royal Statistical Society paper will be published in 2006).
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