Chapter 12.3, Problem 45E

A Cereal-Marketing Survey A food distribution company conducted a survey to determine whether a proposed premium to be included in boxes of its cereal was appealing enough to generate ne\v sales. Four cities were used as test markets, where the cereal was distributed with the premium, and four cities as control markets, where the cereal was distributed without the premium. The eight cities were chosen on the basis of their similarity in terms of population, per capita income, and total cereal purchase volume. The results are shown in the following table.

Percent Change in Average Market Share per Month
	1	+ 18
Test cities	2	+ 15
	3	+7
	4	+ 10
	1	+ 1
	2	-8
Control cities	3	-5
	4	0

Find the standard deviation of the percent change in market share for the control cities.