Epidemiological principles for EMF and EMR studies - Lincoln ...
Epidemiological principles for EMF and EMR studies - Lincoln ...
Epidemiological principles for EMF and EMR studies - Lincoln ...
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8<br />
A case-control study looks back at the time to see what proportion of those who<br />
have the disease were exposed <strong>and</strong> not exposed. They are the then compared with<br />
a group who do not have the disease <strong>and</strong> their proportion of being exposed <strong>and</strong> not<br />
exposed, Figure 1. This creates the classical 2x2 analysis <strong>and</strong> shows the problem<br />
of having no non exposed reference groups.<br />
Figure 1: The basic design of a case-control study, Beaglehole, Bonita <strong>and</strong><br />
Kjellström (1993).<br />
Two-by-Two Tables:<br />
Two common expressions of the comparative incidence is the ratio of ratios, called<br />
the Odds Ratio (OR) <strong>and</strong> the ratio of rates called the Relative Risk or Risk Ratio<br />
(RR). This is set out as a 2x2 table where the exposed group who have the disease<br />
are termed group A, <strong>and</strong> those who have been exposed but do not have the<br />
disease are group B. Group C were not exposed but do have the disease, while<br />
Group D were neither exposed nor have the disease, Table 1.<br />
Table 1: The classic 2x2 table <strong>for</strong>mat <strong>for</strong> a case-control study.<br />
Disease Status<br />
Yes No Total<br />
Exposure Yes A B A+B<br />
Status No C D C+D<br />
A+C B+D N<br />
The Odds Ratio is given by: OR = (A/B)/(C/D) = AD/BC<br />
The proportion of all the people who have been exposed <strong>and</strong> have the disease is<br />
A/(A+B) <strong>and</strong> the proportion of the not exposed people with the disease is C/(C+D).<br />
For the Relative Risk is given by: RR = (A/(A+B))/(C/(C+D))<br />
For example: If A = 30, B = 970, C = 15 <strong>and</strong> D = 985.<br />
Then OR = AD/BC = 30x985/(15x970) = 2.03, <strong>and</strong><br />
RR = (A/(A+B))/(C/(C+D))= (30/(1000))/(15/1000)= 2.0