A food inspector , examamining 12 jars of a certain brand of peanut butter, obtained the following percentages of impurities :
2.3,1.9,2.1,2.8,2.3,3.6,1.4,1.8,2.1,3.2,2.0 and 1.9.
Construct a 90% confidence interval for the standard deviation of the population sampled, that is for the percentage of impurities in the given brand of peanut butter.