Question

Epidemiology: which method of sampling controls results in an odds ratio that is equal to the risk ratio in cohort study?

Answer 1

calculation of OR(ODD RATIO)or RR(RISK RATIO) value, we have to validate with calculation of their Confidence interval (CI) or standard error (SE) and also with Chi square test. And we could argue if a result is significant or not, only if also chi square value is significant and SE > 1.OR is to estimate if a suspicious factor enhances the occurrence of a given disease. This risk estimator is always calculated in case control studies. It's the fraction= Exposition to the factor in case/Exposition to the factor in controls.RR is to estimate if the occurrence of a given disease is much important in exposed group to a given factor. It's = Incidence in exposed group/Incidence in non exposed group.

Odds ratio is the ratio of the odds of disease in the exposed to the odds of disease in the non-exposed,OR = odds that an exposed person develops the disease / odds that a non - exposed person develops the disease Odds ratio can be calculated in a cohort study and in a case control study.Relative risk can only be calculated in a cohort study

When the “cases” studied are representative of all people with the disease in the population from which the cases were drawn, with respect to history of the exposure. When the “controls” studied are representative of all people without the disease in the population from which the cases were drawn, with respect to history of exposure

In medical literature, the relative risk of an outcome is often described as a risk ratio (the probability of an event occurring in an exposed group divided by the probability in a non-exposed group). Certain types of trial designs, however, report risk as an odds ratio. This format is commonly expressed in cohort studies using logistic regression. When the incidence of an outcome is low (<10%), the odds ratio is very similar to the risk ratio.1 However, the odds ratio becomes exponentially more different from the risk ratio as the incidence increases, which exaggerates either a risk or treatment effect

If OR = 1

− Exposure is not related to disease

− No association; independent

If OR > 1

− Exposure is positively related to disease

− Positive association; ? causal

If OR < 1

− Exposure is negatively related to disease

− Negative association; ? protective

Relative Risk or Risk Ratio

Relative risk (RR) = Risk in exposed / Risk in non-exposed

If RR = 1

− Risk in exposed = Risk in non-exposed

− No association

If RR > 1

− Risk in exposed > Risk in non-exposed

− Positive association; ? causal

If RR < 1

− Risk in exposed < Risk in non-exposed

− Negative association; ? protective.