It is known that some brands of peanut butter contain impurities within an acceptable level. A test conducted on randomly selected 12 jars of a certain brand of peanut butter resulted in the following percentages of impurities:
1.9 2.7 2.1 2.8 2.3 3.6 1.4 1.8 2.1 3.2 2.0
(a) Construct a 95% con?dence interval for the average percentage of impurities in this brand of peanut butter.
(b) Give an approximate 95% con?dence interval for the population variance.
(c) Interpret your results and test for normality.