Question

. A business researcher wants to estimate the average number of years of experience an account manager has working with the company before getting promoted to account manager. Eight account managers are randomly selected and asked how long they worked with the company before becoming an account manager. The resulting answers were:     1.2, 4.0, 3.6, 0.7, 5.8, 3.3, 2.8, 4.1

      Use Excel and these data to compute a 90% confidence interval to estimate the average length of time an account manager spent working for the company before they were promoted to account manager. Print out your answer.

      Directions:

For most current versions of Excel, go to the Data tab, select Data Analysis, and Descriptive Statistics. Enter the input range. Check off that you want Summary Statistics and Confidence Interval for the mean. Enter the value of the level of confidence. In the output you will get several things. The value of the mean is the point estimate. The value labeled “confidence level” is actually the + error of the interval. The “error of the interval” already has the table value of t and the standard error of the mean computed within it. Use the mean and the “error of the interval” to make the confidence interval. You will probably have to manually type this out somewhere on the spreadsheet or cut and paste it together in Excel and printout the confidence interval.

Answer 1

Here we have given that

X: average number of years of experience of account managers

average experience of manager (X)
1.2
4
3.6
0.7
5.8
3.3
2.8
4.1

Now, we want to find 90% confidence interval for estimate the average length of time an account manager spent working for the company before they were promoted to account manager

we take related statistics using excel software

One-sample
N	8
Mean	3.188
stDev	1.638
SE Mean	0.579

Now, we want to find the 90% confidence interval for population mean

Formula is as follows,

Where

E=Margin of error =

Now,

Degrees of freedom = n-1 = 8-1=7

c=confidence level =0.90

=level of significance=1-c=1-0.90=0.10

and we know that confidence interval is always two tailed

t-critical =1.895 ( using t table )

Now,

=1.097

We get the 90% confidence interval for the population mean

Interpretation:

This is the 90% CI which shows that we have 90% confidence that this population mean will fall within this interval.