This was estimated by assuming that the 44 sampled diagnostic centres were representative of the population of 551 centres in terms of the number of tests per centre,
allowing Tdt to be calculated as 14,000 ± 1100. This approach is considered reasonable,
as the 44 sampled centres were found to be quite homogeneous in terms of the number of tests per centre per day (25 ± 2). However, it is likely that 551 is a slight underestimate of the number of centres.