Most Australian youth-sports leagues separate athletes by birthdate, and the cutoff date is January 1st. Thus, those children born in January and February have some physical advantages in youth sports over those born in November and December. A recent study suggests that those physical advantages enjoyed early in life impact the likelihood of a child becoming a professional athlete. Table 7.12 gives the number of Australian-born 2009 Australian Football League players born in different months of the year, as well as the proportion of births expected if the birthdates of the athletes matched the distribution of all births nationally. Which parts of the year have a higher than expected number of AFL athletes? Which have a lower than expected number? Is there evidence that the distribution of birthdates of AFL athletes is not the same as the distribution of birthdates nationally? If there is evidence of a difference between the actual and expected counts, which categories are contributing the most to the sum for the chi-square test statistic?

Table 7.12

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Proportion Nationally Actual for AFL players Months Jan-Mar Apr-June Jul-Sep Oct-Dec 0.248 196 0.251 0.254 162 137 122 0.247