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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

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