# Calculation of incidence rate for epidemiological study — prevalence rate this time

Thanks for all the answer for the question Calculation of incidence rate for epidemiological study in hospital. And here come's the second part of the question:

What about the prevalence rate then? I have read The new public health as suggested by Chi, the book says that prevalence is usually not available when using ordinary incidence rate, but I saw another formula here:

total case count in that period of time/total patient bed days during that period of time

It puzzled me again, what is it? I have never heard of prevalence calculated using denominator as patient-bed days.

Thanks!

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It's a somewhat unusual way to calculate prevalence, but it makes some sense to use the patient-bed days as the denominator. Consider two scenarios:

A hospital has a single patient, who stay for 1,000 days, and in that time, has a single infection.

B hospital has 1000 patients, who stay for 1 day each, and in that time, they have 50 infections.

Using "N" as the denominator:

A Prevalence = 1.00 B Prevalence = 0.05

Using Patient-Days as the denominator:

A Prevalence = 0.001 B Prevalence = 0.05

The latter accurately reflects the higher burden of disease. Generally speaking, when you calculate a prevalence just using N, it's under the assumption that all persons at risk are at risk for the same amount of time. In the example above, that isn't true.

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Person time is mostly used when calculating incidence. That is, when you are taking from a specific point in time to another. It is also called the incidence density. When using the total population as the denominator you will be calculating the cumulative incidence.

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Welcome to the site, @Femi. I edited your answer to make the English smoother. I made some educated guesses regarding what you were trying to say. Please ensure it still says what you want. –  gung Dec 26 '12 at 23:25