The jurisdiction epidemiologist had a randomized sample from all children registered in elementary schools in the jurisdiction, because of this the age and gender distribution is more homogeneous that in sample obtained by the medical doctor, because, this sample was not randomized.

Conclusion. This fact is demonstrating the importance of randomized samples.

Although the number of subjects in each sample is the same (500), the results are different, because the sample of medical doctor has more children with less age (almost 45% had between 6 and 7 years) and they are not representative from all population f children registered in the schools.