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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

Download ConfidenceInterval_StdDev.xlsx

Excel Protected View

When you open downloaded files, Excel displays a Protected View warning. You must click Enable Editing before you can use Quantum XL with the file.

Alternatively, you can copy the sample data from the table below and paste it directly into a new Excel workbook.

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. Launch the analysis

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

2. Select your data

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

3. 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.