Risk Ratio A risk ratio, or relative risk, compares the risk of some health-related event such as disease or death in two groups. The two groups are typically differentiated by demographic factors such as sex (males versus females) or by exposure to a suspected risk factor (e.g., consumption of potato salad or not). Often, the group of primary interest labeled the “exposed”, the comparison group labeled the “unexposed” group. We place the group that we are primarily interested in the numerator; we place the group we are comparing them with in the denominator: Risk Ratio = risk for group of primary interest × 1 Risk for comparison group

Relative Risk (Risk Ratio) Relative risk ( RR), Risk Ratio Estimates the magnitude of an association between exposure & disease & indicates the likelihood of developing the disease in the exposed group relative to those who are not exposed. RR = I.R. in the exposed Ie I.R. in the non exposed Io It measures the strength of the association, which is important in the causation & in the policy of prevention. RR = 1: means zero (no effect of the exposure) RR > 1: means (positive effect of the exposure) RR < 1: means negative effect of exposure (preventive)

Attributable risk (AR), Risk difference: The absolute effect of the exposure or excess risk of the disease in those exposed compared with those none exposed. AR = Ie – Io AR% (Attributable proportion) : refers to the percent of cases which is due or attributed only to the exposure, or: the number of cases which could be prevented if we remove the exposure to that factor. AR% = Ie – Io * 100 Ie

Rate Ratio A rate ratio compares two groups in terms of incidence rates, person-time rates, or mortality rates. Like the risk ratio, the two groups are typically differentiated by demographic factors or by exposure to a suspected causative agent. The rate for the group of primary interest is divided by the rate for the comparison group. Rate ratio = rate for group of primary interest × 1 rate for comparison group The interpretation of the value of a rate ratio is similar to that of the risk ratio.

Odds ratio An odds ratio is another measure of association which quantifies the relationship between an exposure and health outcome from a comparative study. The odds ratio is calculated as: Odds ratio = ad bc a = No. of persons with disease and with exposure of interest b = No. of persons without disease, but with exposure of interest c = No. of persons with disease, but without exposure of interest d = No. of persons without disease & without exposure of interest a + c = total number of persons with disease (“cases”) b + d = total number of persons without disease (“controls”)