Veterinary Epidemiology and Zoonosis

Ratio, rate and proportions

EXERCISE-3: MEASUREMENT OF DISEASE DETERMINANTS OF MORBIDITY AND MORTALITY RATES/RATIOS

Ratio

• It is a relative magnitude of disease events that means the events are mutually exclusive.
• Ratio is a relation between numerator and denominator in which the former is not included in the later.

Rate

• A rate is a ratio that expresses a change in one quantity (numerator) with respect to another quantity (denominator).
• Time is usually included in the denominator. Example: wind velocity is 10 m/sec. Epidemiological rate is commonly expressed as 10^a, where ‘a’ may be 2 to 6.

 

Rate

=

No. of individuals having the characteristic disease during the time interval of observation   ----------------------------------------------------------------------------------------------------------------------------------------------------  (No. of individuals at risk of having or getting the characteristic disease during the same interval of observation) x (Length of interval of observation)

x

10a

Proportions

• If the ratio is expressed incorrectly as rates in which the numerator is a subset of the denominator (they are not true rates).
  • Example: cumulative incidence rate, case fatality rate, foetal death rate, prevalence rate, survival rate etc.