26. Harrison et al. (A-29) conducted a study of dependent elderly people in a London borough. Along with other characteristics, they collected data on the extent of depression among borough residents. In a sample of 158 subjects who had a previous diagnosis of depression, 48 were rated during the survey as having depression. In a sample of 745 subjects with no previous diagnosis of depression, 311 were rated by the survey as having depression. Construct a 99 percent confidence interval for the difference between population proportions. State the assumptions that make your procedure valid.