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Created: Aug 30, 2019
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Aggression Levels
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   Lead can be toxic and can retain in the body over a long period of time resulting in cumulative poisoning (Britannica, 2019). Children that are exposed to lead can experience cognitive deficits that could potentially lead to aggression. In this study researchers want to determine, “are boys with high levels of lead more aggressive than boys with low levels of lead?”

Data / Methods

  Data was collected from two-hundred boys attending public schools in Pittsburgh, each boy being categorized as having low or high levels of lead in their bones. A 4-point scale on degrees of aggression was used (1=low, 2= moderate, 3=high and 4=extreme). The data was then analyzed using a Mann-Whitney Test for ordinal data. The distribution of data between the two groups have similar shapes and spreads, all conditions are valid.


   At the 0.05 significance level, one must find that there is a lack of evidence of any difference in the medians between aggression in boys having low levels of lead in their bones and boys having high levels of lead in their bones.

Mann-Whitney Test

Hypothesis test results:
m1 = median of Low
m2 = median of High
m1-m2 : m1 - m2
H0 : m1-m2 = 0
HA : m1-m2 ≠ 0




Diff. Est.

Test Stat.



m1 - m22






Norm. Approx.



    Our analysis and statistical results showed that there is no difference and a lack of evidence showing that boys having low levels of lead in their bones and boys having high levels of lead in their bones leads to aggression. Limitations to this study, there is no indication that “intelligence” was tested prior to this study. Researcher suggested that lead can lead to cognitive deficits over time but it does not state if any of the participants already have cognitive issues. There is also no data showing the age of the two-hundred participants, there could different cognitive development between ages that may affect aggression in boys.


    The researcher study “Aggression Levels”, does not show that effects of lead in children’s blood, bones and tissue will cause aggression in boys. Researchers make it clear that effects of lead in children can lead to cognitive deficits, this could potentially be a better research question to ask. Do the effects of lead in children’s blood, bones and tissue lead to cognitive deficits in boys? This is also one of the large limitations in the study as researchers do not state if cognitive deficits were examined prior to data collection. Example, if a child is severely autistic, would their already aggressive behavior skew the results drastically?


Britannica. (2019). Lead. Encyclopedia of Britannica. Retrieved From

HTML link:
<A href="">Aggression Levels</A>

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By deanna.curry107
Aug 31, 2019

I received a p-value of 0.001 which led me to believe that there was a difference in the aggression of the boys with the higher levels. My son was diagnosed with very high lead levels at six years old and we could not figure out why. Some of the confounding variables that were addressed was if he ate paint chips, put toys in his mouth, and what does he eat. I had to complete a chart everyday between his appointments of every two weeks to see what the doctors could eliminate for reasoning of him getting the lead. It was so difficult to deal with because it had really affected him being the normal kid he had been. About a month after him going to the Lead Clinic and attending all of these visits the specialists had concluded that his hot wheels were a big factor in which I had to take away and his lead levels had began to decline. I could not believe the outcome, but was glad we figured out the problem.
By walkerj10
Aug 31, 2019

I would have focused more on the p-value since it was so small which concludes on your alternative hypothesis (research question).
By nku.dr.nolan
Aug 31, 2019

Hi Sarah - some comments on a couple important points:

1. Your p-value is small, less than 0.05, so this should lead to concluding your research hypothesis, namely that median level of aggression is higher in the high-lead group. It also looks like you did a two-sided test, but this research question is one-sided.
2. Note that if it were correct to conclude "lack evidence" this is not the same as what you stated in your discussion when you say there "is no difference". Lack of evidence that there is a difference does not mean that there is not one (perhaps our sample size was just too small to investigate).

Hope this helps!
By brian.hargis1337
Aug 30, 2019

My initial reaction to this is that there are too many variables to simply only test bone toxicity levels and derive and conclude causality for related aggression levels via interrupted cognitive development, especially since you noted there was a lack of establishing values and pre-testing. I also feel like this number participants are too small, and the sample site being only one school (in the same city with the same potential water supply) lacks any comparable data from other demographics, ultimately not allowing for a trend to emerge.

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