One Variance

Confidence intervals - Chi-square method

Use the chi-square method if you can safely assume that your sample comes from a normally distributed population. These two confidence intervals provide ranges of values that are likely to contain the unknown population standard deviation and variance, respectively.

Example Output

Method

The chi-square method is only for the normal distribution.

The Bonett method is for any continuous distribution.

Statistics

Variable N StDev Variance

Length 50 0.871 0.759

95% Confidence Intervals

CI for

Variable Method CI for StDev Variance

Length Chi-Square (0.728, 1.085) (0.529, 1.178)

Bonett (0.704, 1.121) (0.496, 1.257)

Interpretation

In the lumber yard example, if the data are normal, then the standard deviation of the length of all beams from the lumber yard is likely to be between 0.728 cm and 1.085 cm. The variance of that length is likely to be between 0.529 cm $image\squared.gif$ and 1.178 cm $image\squared.gif$ .