Six Sigma Tutorial -> t Confidence Interval for a Variance

Six Sigma t Confidence Interval for a Variance

Use the c2  (chi-squared) distribution for the confidence interval for the variance

The confidence interval (C.I.) includes the area under the curve in between the critical values, excluding the tail areas (the α risk). The entire curve represents the most likely distribution of population variances (sigma squared), given the sample's size and variation.
 

Six Sigma t Confidence Interval for a Variance Example

Calculate a 95% C.I. on variance for a sample (n = 35) with an S of 2.3"
 

This interval represents the most likely distribution of population variances, given the sample's size and variance. 95% of the time, the population's variance will fall in this interval