Question

In a hypothetical study, the investigator wants to look at the association between exposure to pesticides in the workplace and the development of asthma. The investigator selects a large group of individuals who work on PLTW farms across the country and follow them over ten years to determine who develops asthma. Of the 318 individuals exposed to pesticides, 69 develop asthma. 990 individuals worked in areas that were free of exposure to pesticides. 903 are asthma-free after ten years.

Is the hypothetical study a prospective or retrospective cohort study? Explain your answer.



Create the corresponding data table to show the results of this study.






Calculate the appropriate measure of association for this data. Show your work.





Interpret your results. What does this measure of association tell you about the development of disease?




Describe a weakness of this study design. How does this weakness affect overall reliability of results?

Answer 1

1) Prospective cohort study. 2) Table below. 3) Relative risk = 2.466. 4) The risk of developing asthma is greater in people exposed to the pesticide than in people not exposed to it. 5) Not knowing other risk factors that could cause asthma.

What are prospective and retrospective cohort studies?

Prospective and retrospective studies are carried out depending on the aim of the research.

• Retrospective studies analyze the reason for which something occurred or explain an event or recent discovery.

Individuals from the study groups in these cases have already been affected.

• Prospective studies are carried out to predict a future event.

Most individuals from the study groups in these cases have not been affected. However, some of them might have been or might have high risks of being affected.

1)

The exposed study case is an example of a prospective cohort study.

It analyzes the connection (relationship) between being exposed to the pesticide and being affected by asthma.

It is carried out to predict if people exposed to pesticides have more chances of developing asthma or not.

2) Data

• Total number of individuals exposed to pesticides: 318
• Individuals who developed asthma: 69
• Individuals who did not develop asthma: 249

• Total number of individuals not exposed to pesticides: 990
• Individuals who developed asthma: 87
• Individuals who did not develop asthma: 903

Pesticide Expossure          Asthmatic        Not Asmathic     Total

Yes                                            69                       249               318

No                                              87                       903               990

Total                                          156                      1152              1308

3) Measures of association

Exposed people

attack rate = affected people with asthma / exposed people

attack rate =  69/318

attack rate = 0.217

Attack rate for peope exposed to pesticide, ARE = 0.217

Not Exposed people

attack rate = affected people with asthma / not exposed people

attack rate =  87/990

attack rate = 0.088

Attack rate for peope not exposed to pesticide, ARNE = 0.088

Being exposed

Relative risk = ARE / ARNE

Relative risk = 0.217 / 0.088

Relative risk = 2.466

4) Interpretation:

• Relative risk = 1 ⇒ the risk of being affected is the same for the exposed and unexposed groups. Being exposed is not related to being affected.

• Relative risk > 1 ⇒ the risk of being affected is greater for the exposed group than for the unexposed group. Being exposed is closely related to being affected.

• Relative risk < 1 ⇒ the risk of being affected is greater for the unexposed group than for the exposed group.

In the exposed example, relative risk is greater than 1 (Rr>1)

This means that the risk of developing asthma is greater in people exposed to the pesticide than in people not exposed to it.

Pesticide exposure might be a significant risk factor for asthma development.

5) Weakness of this study design  

A weakness of the study could be that no other risk factor is considered.

There could be another factor affecting people and causing asthma development. This factor could be affecting both groups.      

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