Epidemiology module of practical skills for mbbs student

Exercises and Solutions
Rates and Percent Distributions

Exercise A

The following are data for two counties:


Recall the formulas for the crude birth and fertility rates:

For each of the counties, calculate the following:

1. the crude birth rate

2. the crude fertility rate

3. the fertility rate for adolescents aged 10-17

4. the percent of total births to adolescents aged 10-17

Do either County X or County Y have an excess of adolescent births? Which county is in more need of programs targeted at the adolescent population?
SOLUTION:

Rates and Percent Distributions

Solution for Exercise A


County Y has a higher % births to adolescents than County X, but a lower adolescent fertility rate. This is because the adult women in County Y have a lower fertility rate than the adult women in County X resulting in the lower overall birth and fertility rates seen in the above table for County Y. The elevated %births to adolescents in County Y compared to County X, therefore, is due to these differences in the rates for the adult women.
The % births to adolescents are not a rate because adult women who are no longer at risk of delivering in their teen years are included in the denominator. It can be argued that the adolescent fertility rates are a “fairer" comparison of the two counties since they are based solely on the experience of their adolescent populations, unrelated to the experience of other women. On the other hand, some might argue for using% births to adolescents precisely because it reflects the experience of teenage women in relation to the overall fertility behaviour in their communities.
Defining excess adolescent fertility must be based on a community standard. It may be that any adolescent childbearing is considered an excess, or this may be determined in relation to a state or National average or goal.

2. CONCEPT OF NUMERATOR AND DENOMINATOR:

For calculation of rate, ratio or proportion proper understanding of the Concept of Numerator and Denominator is very important.

a. Numerator: Numerator refers to the number of times an event (e.g. number of birth) has occurred in a population, during a specified time period.

b. Denominator: Numerator has little meaning unless it is related to the denominator. The epidemiologist has to choose an appropriate denominator while calculating a rate. It may be related to: (I) population (II) the total events.