Ans. is 'd' i.e., Risk ratio Relative risk (risk ratio)* Relative risk is the ratio of the incidence of the disease (or death) among exposed and the incidence among nonexposedRR =Incidence of disease among exposed--------------------------------Incidence of disease among nonexposed* RR is a direct measure of the strength of the association between the suspected cause and effect.* A relative risk of one indicates no association.* A relative risk greater than one indicates positive association between the exposure and the disease under study.* For example, a RR of 2 indicates that the incidence rate of disease is 2 times higher in exposed group as compared with unexposed -> 100% increase in risk.* A relative risk of less than one indicates negative association, i.e. exposure to a factor will decrease the incidence of disease (e.g. exposure to vit A decreases the incidence of skin cancer).Attributable risk (risk difference)* Attributable risk is the difference in incidence rates of disease (or death) between an exposed group and non- exposed group.AR =Incidence of disease in exposed -Incidence of disease in unexposed--------------------------------------------------------Incidence of disease in exposedx 100* AR is also known as absolute risk or excess risk or risk difference* Attributable risk indicates to what extent the disease can be attributed by risk factor.* For example, AR of 90% means 90 percent of disease (e.g. lung cancer) among exposed (e.g. smoker) is due to exposure to risk factor (smoking).* This suggest the amount of disease that might be eliminated if the factor could be controlled or eliminated, i.e. AR of 90% indicates that there will be 90% reduction in incidence of disease (e.g. lung cancer) if risk factor (smoking) is eliminated.Relative risk Vs Attributable risk* Relative risk is a direct measure of the strength of association between suspected cause and effect. For example a relative risk of 2 means that the incidence rate is 2 times higher in the exposed group as compared with unexposed - i.e., a 100% increase in risk.* On the other hand, attributable risk indicates the extent which is attributed by risk factor (exposure) to disease. For example attributable risk of 90% means 90 percent of disease among exposed is due to exposure to risk factor.* In simple words# Relative risk indicates the increased percentage of risk of developing a disease, if person is exposed to risk factor.# Attributable risk indicates the percentage of disease which is attributed by risk factor among the exposed.* Relative risk is a better index than is attributable risk for assessing the etiological role of a factor in disease.* On the other hand, attributable risk gives a better idea than does relative risk of the impact of successful preventive or public health programme might have in reducing the problem. That means attributable risk reflect the public health importance better than relative risk.Population attributable risk (PAR)* It is the incidence of disease (or death) in the total population minus the incidence of disease (or death) among those who are not exposed.PAR =Incidence in total population in non exposed--------------------------------------Incidence in total populationx 100* The concept of population attributable risk is useful in that it provides an estimate of the amount by which disease could be reduced in that population, if the suspected factor is modified or eliminated.* So, population attributable risk gives a better idea than does relative risk of the impact of successful preventive or public health programme might have in reducing the problem.* Population attributable risk is broader and better than attributable risk.