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CI for Standard Deviation

The CI for Standard Deviation uses the chi-squared distribution to calculate a confidence interval for the population standard deviation, estimating the range of true process variability.

Goal

Calculate a 95% confidence interval for the standard deviation of diameter measurements.

Sample Data

Press Copy for Excel, then paste the table into a blank worksheet.

Diameter
10.02
9.98
10.05
9.95
10.01
10.03
9.97
10.04
9.99
10.00

Each row represents one diameter measurement. The data has very little variation: the CI for standard deviation will show the range within which the true process variability is likely to fall.

Steps

1. Put the data in Excel

   Press Copy for Excel above the table, click cell A1 in a blank worksheet, and press Ctrl+V. The header row lands in row 1 and the 10 data rows in rows 2 through 11.

2. Launch the analysis

   From the Excel ribbon, select QXL Stat Tools → Analysis Tools → Confidence Interval → CI for Standard Deviation.

3. Select your data

   Select cells A1:A11 (the header row plus all 10 data rows).

4. Configure the analysis

   In the Confidence Interval dialog:

   • Data Columns: "Diameter" should be checked
   • Confidence Level: 0.95 (default)

   Click Finish to generate the confidence interval.

Result

Quantum XL creates a whisker chart showing the 95% confidence interval for the standard deviation. The statistics table shows the sample standard deviation along with the lower and upper bounds of the interval.