Excessive lead levels can negatively affect brain functions; lead poisoning is particularly dangerous to children. A study was conducted to find out whether children of battery factory workers had higher levels of lead in their blood than a matched group of children. Each child in the experimental group was matched with a child in the control group of the same age, living in the same neighborhood. Although these children were not a random sample, we can test the hypothesis that the difference is too large to occur by chance if the child from the control group was randomly chosen. The figure shows a histogram of the differences in lead level.

The differences were found by calculating "factory child's lead level minus matched control child's lead level." Lead levels were concentrations in the blood, measured in micrograms per deciliter. A positive difference means the child of a factory worker has a higher lead concentration than the child in the control group. The data consisted of 1 tie (the same value for the child and the matching child), 28 pairs in which the factory worker's child had a higher level, and 4 pairs in which the factory worker's child had a lower level of lead.
Carry out a sign test to determine whether children whose parents are exposed to lead at work have a higher lead level than children whose parents are not exposed to lead at work. Use a significance level of 0.05 to see whether the experimental group had higher levels of lead.

18 16 - 14 5 12 8 6- 4 -10 10 30 50 70 Difference, Experimental minus Control (micrograms per deciliter)