Often called “the cornerstone” of public health, epidemiology is the study of the distribution and determinants of diseases, health conditions, or events among populations and the application of that study to control health problems. By applying the concepts learned in this course to current public health problems and issues, students will understand the practice of epidemiology as it relates to real life and makes for a better appreciation of public health programs and policies. This course explores public health issues like cardiovascular and infectious diseases – both locally and globally – through the lens of epidemiology.

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Understanding Measures of Disease Frequency

This module introduces measures of disease frequency.

Clinical Associate Professor Department of Epidemiology, UNC Gillings School of Global Public Health

Dr. Lorraine Alexander

Clinical Associate Professor, Director of Distance Learning (North Carolina Institute for Public Health) Department of Epidemiology, UNC Gillings School of Global Public Health

In this segment,

we're going to discuss the measure of disease occurrence known as prevalence.

Prevalence measures are one of the most common statistics you'll hear in the news.

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The learning objectives for this segment are as follows.

To define and calculate the measure of prevalence and

be able to interpret prevalence within the context of public health research.

Prevalence is one of the most common epidemiologic

measures you see in our everyday news.

For example, you may see a news headline based on prevalence,

such as the proportion of people in the population that are overweight.

The measure prevalence helps us quantify the proportion of the population with

the specific health outcome.

For example, approximately 12% of the world's total population is obese.

Prevalence is the proportion of a defined population

that has a particular disease or health outcome of interest.

Prevalent cases are existing cases of disease.

These are cases whose disease developed or

was diagnosed before they were identified for the study.

Prevalence is useful to quantify the burden of a health outcome or

disease in the population at a given point or period of time.

Prevalence can also be useful for planning health services.

11% of people over 65 and older in the United States have Alzheimer's disease.

This statistic is an example of a prevalence.

Here's another example.

Malaria is a life-threatening disease caused by parasites that are transmitted

to people through the bites of infected mosquitoes.

Malaria infects 10% of the world's population.

This statistic about Malaria is an example of a prevalence.

To understand prevalence, imagine a bucket partially filled with water.

Each drop represents an existing case of the disease or health outcome.

The capacity of the bucket represents the total population at risk for

the disease or health outcome.

If we look at the bucket at one point in time, the number of drops or

existing cases in the bucket divided by the total number of

drops the bucket could hold is the prevalence.

Prevalence is a proportion.

The numerator is the number of people with the disease or health outcome and

the denominator is the number of people in the total study population.

Note that people with the disease are also counted in the denominator.

Here's a formula for a prevalence.

The numerator is the prevalent cases, all the existing cases at a given point in

time and the denominator is the total study population.

For all measures of disease occurrence,

it is important to think about who should be in the denominator of your calculation.

You want the denominator to represent the people who could have the disease or

health outcome in your study population.

Think about who would be in the denominator when calculating

the prevalence of pregnancy among women in North Carolina.

The denominator would be women of reproductive age in North Carolina

who are able to become pregnant.

Now, think about who would be the denominator when calculating

the prevalence of prostate cancer in the United States?

This would be men susceptible to prostate cancer in the United States.

The other important component of the measure of prevalence

is the time period specified.

It is important to specify the time point or

period over which the prevalence measure is calculated.

For example, a year is a common time period used for prevalence.

Next, we will work through an example of how to calculate prevalence.

On December 1, 2009, all residents in one town were surveyed.

So, the total population at this state was 14,000.

The residents were asked if they currently had a cold.

200 of the 14,000 reported having a cold.

What is the prevalence of cold on December 1, 2009, in this population?

So, looking at the formula for

prevalence, we note that we need to find the number of prevalent cases and

then divide it by the total number of individuals in the study population.

In this example, we look for the number of prevalent cases of cold.

In the town, there were 200 prevalent cases of cold reported.

The denominator in the population of residents is 14,000,

thus the prevalence of cold in the town on December 1st, 2009 was 1.4%.

Prevalence is often referred to as a cross-sectional measure, because it tells

us about the number of people with the health outcome at one slice in time.

Now, we will discuss two types of prevalence calculations.

Remember, prevalence can be viewed as a slice through the population

at a point in time in which it is determined who has the disease or

health outcome, and who does not.

The two types of prevalent measures are point prevalence,

often just called prevalence, and period prevalence.

Point prevalence is the prevalence at one point in time.

Period prevalence is the number of people with a health outcome

over a specified period of time,

divided by the number of people in the population during that time period.

In this example, it's the summer of 2012.

An example of point prevalence is, do you currently have asthma?

In contrast, a period prevalence example would be,

have you had asthma during the past three years?

The time period is the past three years.

So, you can see here, the only difference between point and

period prevalence is the time period specified.

We have seen in the previous slides how important the time period is

when determining a prevalence.

In the example on this slide, we are presented with two prevalence statistics.

Think about which prevalent statistic is of greater concern.

What is the time frame?

Are we talking about new or old cases?

Is all the needed information provided?

Maybe, the Alamance County cases occurred over the past century.

These cases may be anyone who ever had rabies over the past 100 years.

So, the concern for today may be minimal.

What if the global measure is 10 new cases of rabies in a day?

This number isn't really high, but we would be more concerned about new cases

of disease than a historical record of existing cases of disease.

So, we're comparing apples and oranges in this example.

We need to be specific about what information we're reporting to avoid this

confusion in the future.

We will go through another example to illustrate how to calculate the measure

of prevalence.

In this example,

500 men with lung cancer are asked to report on their smoking habits.

Of the 500 men, 461 reported smoking at least one pack of cigarettes a day.

How do you calculate the prevalence of smoking among men with lung cancer

enrolled in this study?

The numerator is the number of smokers, or 461.

The denominator is the total number of men with lung cancer in the study, or 500.

The prevalence is equal to 461 divided by 500,

or 0.922, or 92.2%.

The interpretation is the prevalence of smoking among

these 500 men with lung cancer is 92.2%.

Now, we'll give you the opportunity to calculate a prevalence.

Now, we've covered the basic definition of prevalence.

The most important thing to remember about the definition of prevalence

is what's in the numerator and what's in the denominator.

So, for prevalence, the numerator is all existing cases of disease.

That includes both new and old cases, and

in the denominator is the total population.

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