The following table contains information on matched sample values whose differences are normally distributed. (You may find it useful to reference the appropriate table: z table or t table)

Number	Sample 1	Sample 2
1	19	20
2	11	13
3	22	21
4	23	20
5	16	20
6	14	16
7	18	18
8	17	22

Let the difference be defined as Sample 1 - Sample 2.

a. Construct the 99% confidence interval for the mean difference _D. (Negative values should be indicated by a minus sign. Round intermediate calculations to at least 4 decimal places and final answers to 2 decimal places.)