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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