Question

A random sample of 28 statistics tutorials was selected from the past 5 years and the percentage of students absent from each one recorded. The results are given below. Assume the percentages of students' absences are approximately normally distributed. Use Excel to estimate the mean percentage of absences per tutorial over the past 5 years with 90% confidence. Round your answers to two decimal places and use increasing order.

Number of Absences

13.9

16.4

12.3

13.2

8.4

4.4

10.3

8.8

4.8

10.9

15.9

9.7

4.5

11.5

5.7

10.8

9.7

8.2

10.3

12.2

10.6

16.2

15.2

1.7

11.7

11.9

10.0

12.4

Answer 1

Enter the data of 28 values. Sample size, n =28

Excel Formulae:

=AVERAGE(A2:A29) - mean (in cell A30)

=STDEV(A2:A29) - std.deviation (in cell A31)

=SQRT(28) - square root of sample size (in cell A32)

=A31/A32 - std. error (in cell A33)

=TINV(0.1, 27) - t-critical value at df =n-1 =27 and at 90% confidence level for a two-tailed case (in cell A34)

=A33*A34 - Margin of Error (in cell A35)

=A30 - A35 (Lower bound of 90% confidence interval) (in cell A36)

=A30 + A35 (Upper bound of 90% confidence interval) (in cell A37).

Thus, 90% confidence interval for the population mean percentage of absences =(9.22, 11.61)