In common with all Public Sector Comparators, GCHQ's estimates of the costs of the public sector comparator are subject to very great uncertainty. Such uncertainty is inevitable in forming estimates of alternative means of delivering a major 30 year programme of building works and Information Technology development. In line with Treasury guidance, GCHQ undertook a number of sensitivity tests on the costs of the two site public sector comparator to examine the extent of the uncertainty. All the following costs are Net Present Values.