Veterinary Epidemiology and Zoonosis

Measurement of causal association

EXERCISE-6: MEASUREMENT OF CASUAL ASSOCIATION

Relative Risk / Risk Ratio (RR)

• It gives an idea of comparative vulnerability of group to exposure.
• It is the mean of strength of association. Relative risk can be determined by comparing the incidence or prevalence of an event in an exposed and unexposed group.
• If incidence value and prevalence value are identical, then the groups having the identical chance of being exposed.
• Then the relative risk value is one. If the relative risk value is one, there is no association with the cause and effect.
• If the value of relative risk is more than one, will accordingly indicates positive association and if it is less than one, indicates the negative association between cause and effect.  

Group	Disease		Total
	Present	Absent
Exposed	a	b	a+b
Not exposed	c	d	d
Total	a+c	b+d	a+b+c+d=1

• Incidence of disease in exposed population = a / a+b
• Incidence of disease in unexposed population = c / d+c

Odds Ratio

• It is the measure of comparing the cross product of 2x2 contingent table. It gives the odd of getting disease in an unexposed group.

Odds ratio = ad / bc

• Ratio between odds of getting disease in exposed group to the odds of getting the disease in unexposed group.
• If the disease incidence is rare, odds ratio and relative risk will be equal.

Attributable Risk / Ratio (AR)

• It is the rate of events in the exposed group minus unexposed group. Risk of occurrence of an event to a specific cause is attributable ratio.

• AR studies the comparative amount of disease to determine the quantum of disease that is actually attributable to exposure. If the value of the RR is given then

Attributable Factor Percentage (AF %)

• It is the estimate of quantum or amount of an event in an exposed group due to specifically to a factor.

• AF represents the percentage attributable to a particular factor