# What would be an example of an exposure ratio threshold?

What would be an example of an exposure ratio threshold?
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Step 1
Exposure ratio is relevant in a case control study. It involves the following concepts:
1)case group - group of subjects under active study who are exposed to a risk factor.
2)control group - group of subjects which is used as a reference to compare the results of risk on the case group.
3)risk factor - manifestation of a risk (e.g., a disease) in subjects of control group or case group.
Exposure ratio is defined as the ratio of:
the rate at which persons in the case group are exposed to a risk factor to
the rate at which persons in the control group are exposed to the risk factor under study.
Exposure ratio threshold can be defined as the value of exposure ratio above which we can conclude that the risk factor is significant contributor to the effect witnessed in the case group vis a vis the control group.
Step 2
Considering an example:
Case group consists of 30 healthy people having blood group 0
Control group consis of 30 healthy people having a mix of all other blood groups.
Both groups are exposed to polluted air with high particulate matter for two hours everyday for 30 days.
At the end of the experiment , number of people from both groups suffering from respiratory diseases are identified.
Exposure ratio in this case will be ratio of number of sick people in case group to the number of sick people in the control group.
If this value is greater than a specified threshold (exposure ratio threshold) then we can conclude that people with blood group O are more vulnerable to diseases caused by polluted air.
Exposure ratio $=\frac{\text{rate of case group positives}}{\text{rate of control group positives}}$
Step 3
A sample scenario is presented for application of exposure ratio and the threshold value.