I recently surveyed 200 organisations to establish a number of key facts about them including the number of staff. 160 of them responded with the staff information (10,000) and I used the mean staff figure (62.5) for these 160 organisations to estimate the number of staff in the remaining 40 organisations (2,500). This gave me a total estimate of staff in the 200 organisations of 12,500. I'm wondering how I go about working out a 95% confidence interval for the 12,500 figure?