You want to test the hypothesis that the sample of murderers was drawn from a population where the probability of being XYY is .005. You then calculate the probability of observing 2 out of 7 men with an XYY chromosome combination, under the assumption of the null hypothesis that the population probability is .005. If this probability is less than our chosen alpha of .001, we will reject the null hypothesis. This is less than .001, so we reject the null hypothesis. The probability of having an XYY chromosome set for those convicted of murder is greater than .005.
White citizens are more satisfied with their police services than black citizens.
The test statistic is the Mann-Whitney U test. In order to find the critical value of U , we look at the Mann-Whitney U table. The critical value of U is 20. Remember that values equal to or lower than the critical value are necessary to reject the null hypothesis. In order to determine these values, we need to assign ranks to the raw scores.