Question

1. The following data on the average time, in minutes, spent per user per month from January to June of one year for a sample of 15 shoe and apparel retail websites:

13.3  9.0  11.1  9.1  8.4  15.6  8.1  8.3  13.0  17.1  16.3  13.5  8.0  15.1  5.8  

Type the above data in column 2 and title the column “Mins on Website” in MINITAB worksheet and answer the following questions. Round answers to two decimal places for questions a-d.

1. Check to see if all conditions are met by doing a probability plot and boxplot.  If conditions are not met, do not do parts b and c.
2. Construct the 95% Confidence interval about the Population Mean number of minutes spent on these websites.  Interpret the confidence interval.  Use MINITAB to compute the confidence interval.  (Instructions above. Copy and paste the MINITAB output found in the Session window after your answer.)
3. Calculate the 90% Confidence Interval about the Population Mean number of minutes spent on these websites.  Interpret the confidence interval.  Use MINITAB to compute the confidence interval.  (Copy and paste the MINITAB output found in the Session window after your answer.)
4. As you increase the level of confidence, how will this affect the precision (i.e., the width) of the interval? (Note:  The smaller the interval, the more precise it is.)

Answer 1

(a) The plots are:

All conditions are met.

(b) The Minitab output is:

The 90% Confidence Interval about the Population Mean number of minutes spent on these websites is between 9.450 and 13.443.

(c) The Minitab output is:

The 90% Confidence Interval about the Population Mean number of minutes spent on these websites is between 9.807 and 13.086.

(d) The width of the interval increases.

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