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The article “Human Lateralization from Head to Foot: Sex-Related Factors” (Science, 1978: 1291–1292) reports for both a sample of right-handed men and a sample of right-handed women the number of individuals whose feet were the same size, had a bigger left than right foot (a difference of half a shoe size or more), or had a bigger right than left foot.
Does the data indicate that gender has a strong effect on the development of foot asymmetry? State the appropriate null and alternative hypotheses, compute the value of , and obtain information about the P-value.
Solution
VerifiedTo test whether or not there is any effect of gender on the development of foot asymmetry, we will use the chi-square test of homogeneity (since the samples have been taken from two separate populations of men and women).
Let be the probability of falling into the category described by the -th row and -th column of the given table (for instance, is the probability that a randomly selected man would have left foot bigger than right foot), for all and
We want to test:
at significance level (our choice).
More informally, the above hypotheses can be stated as
Gender and the development of foot asymmetry are not related
Gender and the development of foot asymmetry are related.
First we need to make sure the assumptions required for carrying out the chi-square test are met, i.e. we need to make sure that at least of cells (i.e. at least i.e. at least 5 cells) have expected counts greater than 5.
Let be the observed count in row and column let
be the row total and let be the column total, for every
and
Let also be the total sample size (i.e. the "total of totals").
The expected counts, denoted by are calculated as
So, for instance, the expected count in the top left cell is
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