MEASURES IN EPIDEMIOLOGY

The objectives of studying epidemiology are to:

Calculate* and interpret the following statistical measures:
— Ratios
— proportions
— Incidence rates, including attack rate
— Prevalence
— risk ratio, rate ratio, odds ratio, attributable proportion

Ratio & Proportion:

General Formula:

The basic formula is as follows:


Numerator(x)
Measure = _______________

Denominator(y)

Rate: The basic formula for a rate is as follows:



Number of cases or events occurring during a given time period
Rate = ___________________________________________________

Population at Risk during the same time period
Risk Ratio:

The formula for Risk Ratio is:

Risk for Group of primary interest
RR = ______________________________

Risk for Comparison Group

Incidence Rate

The formula for incidence rate is:



# new cases of a specified disease reported during a given time interval
IR = __________________________________________________________

Average population during time interval

Attack Rate

The formula for attack rate is:


# new cases of a specified diseases reported during an epidemic period
AR = _________________________________________________________
Population at start of The epidemic period

Example: during the first 9 month of national surveillance for poliomyelitis CDC received 1.068 cases reports which specified sex; 893 cases were in females, 175 in males

How to calculate the female –to male ratio for polio.

1. define x and y: x= cases in females
y= cases in males

2. Identify x and y: x= 893
Y= 175

3. Set up the ratio: x/y = 893/175

4. Reduce the fraction so that either x 0r y equals 1: 893/175=5.1 to 1

Thus there were just over 5 female polio patients for each male polio patient reported to CDC.

Descriptive epidemiology deals with the questions: Who, What, When, and Where. Descriptive epidemiology describes the occurrence of disease in a population.

Analytic epidemiology deals with the remaining questions: Why and How. Analytic epidemiology compares one group of people to another group of people to help determine the cause of a disease.

Epidemiologists use analytic methods to help identify the cause of disease. The results of descriptive epidemiology may present some questions as how or why disease is occurring in certain population groups. Identifying the cause of disease in groups typically involves designing a study to test the hypotheses that were developed using descriptive epidemiology.