BA 275 Statgraphics Winter 2007 Confidence Interval Estimation

Confidence Interval Estimation Using SG

1. To estimate  (population mean) using SG:  With only the summary statistics but no raw data (useful for most of the textbook examples): Click Describe / Numeric Data / Hypothesis Tests... . Check “Normal Mean” and fill in Sample Mean, Sample Sigma (i.e., sample standard deviation) and Sample Size. Ignore the “Null Hypothesis” box.

Ignore it for confidence interval estimation

 With raw data (you will most likely to use this for the computer project): Click Describe / Numeric Data / One-Variable Analysis... . Enter the variable of interest. Click OK. Click the Tables icon and check the Confidence Intervals box. Click OK.

2. To estimate the difference between two population means, 1 – 2, using SG:  With only the summary statistics but no raw data (useful for most of the textbook examples): Click Compare / Two Samples / Hypothesis Tests.... Click “Normal Means” and fill in Sample Mean, Sample Sigma and Sample Size of each sample. Ignore the “Null Hypothesis” box.  With raw data (you will most likely to use this for the computer project):  Choose Compare / Two Samples / Independent Samples (two samples are selected independently.)

 If each sample is stored in one column (e.g., salaries for male employees are in one column and salaries for female employees are in another column), then the “Two Data Columns” button should be checked. Enter one sample as Sample 1 and the other sample as Sample 2.

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 If data are recorded in one column (e.g., Salary) but are distinguished via a code column (e.g., Gender), then the “Data and Code Columns” button should be checked.

 Click Compare / Two Samples / Independent Samples. Enter the two samples of interest. Click OK. The pane named “Comparison of Means” contains the confidence intervals. If the pane is not shown, click the Tables icon and check “Comparison of Means.”

Use the right mouse bottom for more options. For example, you can change the confidence level from the default 95% to 90%.

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3. Selecting a subset of a sample for estimation:  For example, you many want to estimate the mean age of employees with only training level A, or  You may want to compare the difference in the mean salary between the male and female employees who have 6 or more years of work experience.  At the time of entering the variables for analysis, you need to enter the selection in the (Select:) box as shown in the following figures.

 Use the symbol & for AND-operation. For example, type in the (Select) box Training_Level = “A” & Experience > 6 to select a subset of employees with training level A and with 6 or more years of work experience.

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