PharmD Info

Epidemiology is the study of the patterns, causes, and effects of diseases in populations. One of the key components of epidemiology is the use of various measures to describe the occurrence of disease. Some of the most commonly used measures in epidemiology include the proportion, incidence rate, secondary attack rate, point prevalence, and period prevalence. In this article, we'll discuss each of these measures and provide examples of how to calculate them.

Proportion (Attack Rate or Risk)

The proportion, also known as the attack rate or risk, is a measure of the proportion of people who develop a disease during a specified time interval. To calculate the proportion, we need to know the number of cases and the total population at risk during the specified time interval. The formula for calculating the proportion is:

Proportion = Number of cases / Total population at risk during specified time interval x multiplier

For example, let's say that during an outbreak of influenza in a population of 1000 people, 200 people develop the disease. To calculate the proportion, we would use the following formula:

Proportion = 200 / 1000 x 100
Proportion = 20%

This means that 20% of the population developed influenza during the specified time interval.

Incidence Rate (or Person-Time Rate)

The incidence rate, also known as the person-time rate, is a measure of the rate at which new cases of a disease occur in a population over a specified time interval. To calculate the incidence rate, we need to know the number of new cases and the amount of time that each person was at risk of developing the disease during the specified time interval. The formula for calculating the incidence rate is:

Incidence Rate = Number of new cases / Summed person-years of observation during specified time interval x multiplier
(OR) Incidence Rate = Number of new cases / Average population during specified time interval x time

For example, let's say that during a one-year period, 1000 people were under observation for a specific disease. During this time, 50 new cases of the disease were identified. The total person-time observed was 900 person-years. To calculate the incidence rate, we would use the following formula:

Incidence Rate = 50 / 900 x 100,000
Incidence Rate = 5,556 per 100,000 person-years

This means that there were 5,556 new cases of the disease per 100,000 person-years of observation.

Secondary Attack Rate

The secondary attack rate is a measure of the proportion of contacts of a known case who develop the disease. To calculate the secondary attack rate, we need to know the number of new cases among contacts and the total number of contacts. The formula for calculating the secondary attack rate is:

Secondary Attack Rate = Number of new cases among contacts / Total number of contacts x 100

For example, let's say that during an outbreak of measles, there were 20 confirmed cases. Out of the 200 contacts of these cases, 10 developed the disease. To calculate the secondary attack rate, we would use the following formula:

Secondary Attack Rate = 10 / 200 x 100
Secondary Attack Rate = 5%

This means that 5% of the contacts of known cases developed the disease.

Point Prevalence

The point prevalence is a measure of the proportion of individuals in a population who have the disease at a specific point in time. To calculate the point prevalence, we need to know the number of existing cases at a specific point in time and the population at risk at the same specified point. The formula for calculating the point prevalence is:

Point Prevalence = Number of existing cases at a specific point in time / Population at risk at the same specified point in time x 100

For example, let's say that on January 1st, 2022, a population of 10,000 people had a particular disease, and 500 of them were already diagnosed. To calculate the point prevalence, we would use the following formula:

Point Prevalence = 500 / 10,000 x 100
Point Prevalence = 5%

This means that 5% of the population had the disease on January 1st, 2022.

Period Prevalence

The period prevalence is a measure of the proportion of individuals in a population who have the disease during a specific period of time. To calculate the period prevalence, we need to know the number of existing cases during the specified time interval and the average or mid-interval population. The formula for calculating the period prevalence is:

Period Prevalence = Number of existing cases during specified time interval / Average or mid-interval population x 100

For example, let's say that during the year 2022, a population of 100,000 people had a particular disease, and 5000 of them were diagnosed during this time period. To calculate the period prevalence, we would use the following formula:

Period Prevalence = 5000 / 100,000 x 100
Period Prevalence = 5%

This means that 5% of the population had the disease during the year 2022.

In conclusion, epidemiology uses various measures to describe the occurrence of disease in populations. These measures include the proportion, incidence rate, secondary attack rate, point prevalence, and period prevalence. By understanding these measures and how to calculate them, epidemiologists can better understand the patterns and effects of diseases in populations and make informed decisions about public health interventions.