A population is composed of 100 items arranged in some order. Every stratum of 10 items in the order of arrangement tends to be similar in its values. An "every 10th" systematic sample is selected. The first item, randomly chosen, is the 6th item, and its value is 20. The following items in the sample are, of course, the 16th, the 26th, etc. The values of all items in the systematic sample are as follows: 20, 25, 27, 34, 28, 22, 28, 21, 37, 31. Give a 90% confidence interval for the population mean.