Paper 3, Section I\mathbf{I}, H

Statistics
Part IB, 2009

In a demographic study, researchers gather data on the gender of children in families with more than two children. For each of the four possible outcomes GG,GB,BG,BBG G, G B, B G, B B of the first two children in the family, they find 50 families which started with that pair, and record the gender of the third child of the family. This produces the following table of counts:

First two children Third child BB Third child GG

GG1634GB2822BG2525BB3119\begin{array}{ccc}G G & 16 & 34 \\ G B & 28 & 22 \\ B G & 25 & 25 \\ B B & 31 & 19\end{array}

In view of this, is the hypothesis that the gender of the third child is independent of the genders of the first two children rejected at the 5%5 \% level?

[Hint: the 95%95 \% point of a χ32\chi_{3}^{2} distribution is 7.81477.8147, and the 95%95 \% point of a χ42\chi_{4}^{2} distribution is 9.4877.]9.4877 .]