Recent studies have found that American children are more obese than in the past

Question

Recent studies have found that American children are more obese than in the past. The amount of time children spent watching television has received much of the blame. A survey of 100 ten-year-olds revealed the following with regard to weights and average number of hours a day spent watching television. We are interested in testing whether the mean number of hours spent watching TV and weights are independent at 1% level of significance. Referring to Table 11-13, the degrees of freedom of the test statistic are: 2 1 4 9

Explanation / Answer

For a table with r rows and c columns, the number of cells that can vary is (r-1)(c-1). And that's the formula for the degrees for freedom for the chi-square test of independence! The degrees of freedom then define the chi-square distribution used to evaluate independence for the test.

So, here:

(3 - 1) * (3 - 1) = 4

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