11. A common test of short-term memory requires participants to repeat a random string of digits that was presented a few seconds earlier. The number of digits is increased on each trial until the person begins to make mistakes. The longest string that can be reported accurately determines the participant’s score. The following data were obtained from a sample of n 9 participants. The scores are 4, 3, 12, 5, 7, 8, 10, 5, 9.

a. Compute the mean and variance for the sample.

b. Use the data to make a point estimate of the population mean.

c. Make an 80% confidence interval estimate of .