流行病学通常被称为公共卫生的“基石”，它是一门研究疾病的分布和决定因素，健康状况，或人群间的活动和应用于控制健康问题的学科。由于流行病学与现实生活息息相关，并更好地评估公共卫生项目和政策，学生将理解流行病学的研究方法，通过这一门课所学到的理论知识应用到当今的公共健康问题。本课程通过流行病学的视框，探讨了心血管疾病和传染病等公共卫生问题，对地区情况和全球情况都进行了讨论。 翻译: Yi Zhou

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From the course by 北卡罗莱纳大学教堂山分校

流行病学：基础公共卫生科学

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流行病学通常被称为公共卫生的“基石”，它是一门研究疾病的分布和决定因素，健康状况，或人群间的活动和应用于控制健康问题的学科。由于流行病学与现实生活息息相关，并更好地评估公共卫生项目和政策，学生将理解流行病学的研究方法，通过这一门课所学到的理论知识应用到当今的公共健康问题。本课程通过流行病学的视框，探讨了心血管疾病和传染病等公共卫生问题，对地区情况和全球情况都进行了讨论。 翻译: Yi Zhou

From the lesson

Understanding Measures of Disease Frequency

This module introduces measures of disease frequency.

- Dr. Karin YeattsClinical Associate Professor

Department of Epidemiology, UNC Gillings School of Global Public Health - Dr. Lorraine AlexanderClinical Associate Professor, Director of Distance Learning (North Carolina Institute for Public Health)

Department of Epidemiology, UNC Gillings School of Global Public Health

[MUSIC]

Welcome, in this segment, we're going to talk about the measure of rates.

[MUSIC]

The learning objectives we will cover in this segment include.

Defining and calculating the measure of rates.

Define the concept of person-time and be able to apply it to calculations of rates.

And lastly, to interpret rates within the context of public health research.

A rate measure measures the occurrence of new

cases of a health outcome in a population.

A rate is not a proportion because

the denominator is not fixed.

Instead, a rate accounts well for the realistic situation

in which a population is dynamic and changing over time.

Populations at risk change due to changes such as births, deaths, and migration.

In a study population, a person can decide to no longer participate in a study.

Thus, some people may be lost to follow up during the course of study.

A rate takes into account the sum of time, called

person time, that each person remains at risk for that

disease or health outcome under study observation.

In our previous lectures or segments, we

have already learned about prevalence and risks.

Now we will discuss why a rate is a preferred measure to use.

There are important advantages to using a rate

rather than a risk or a prevalence measure.

Rates are more flexible, more exact, and capture

the reality of often having a dynamic changing population.

Rates can also be used to study repeated

events, where a person can develop the health outcome.

They no longer have the disease or health outcome for a period

of time, and then develop the same disease or health outcome again.

The reason we don't use them all the time is that rate data can be more

costly and challenging to collect.

In order to calculate a rate, these are the following steps we use.

First, we must define our study population.

Then determine the number of news cases of the disease or health outcome.

And then finally, specify our denominator, which is the person time at risk.

The formula for rate is as follows.

The rate is the number of new or incident cases

divided by person-time.

Now let's discuss person-time in more detail.

In order to understand how to calculate a rate,

you will need to understand the concept of person-time.

Person-time is the sum of time that each person remains at

risk for the disease or health outcome and under study observation.

Person-time may be expressed in units of person-years, person-months,

person-days, or some other scale. A person in the study can stop

contributing person-time for a variety of reasons.

Such as death, leaving the study, moving to a different country.

Or the person develops the disease or health outcome during the study.

Or the researcher is unable to follow-up with them.

Or the researcher cannot locate the person.

The use of person-time as opposed to just time enables you to handle situations,

in which people die or migrate out of the study population.

Or where there are drop outs in a study, and where you have not been able to

follow your entire study population at risk to

watch for the development of the disease under investigation.

Thus, the follow-up period does not have to be uniform for all participants.

That's an important point to remember. Person-time for a group is the sum

of the times of follow-up for each participant in that group.

Now we'll show you how to actually calculate person-time.

Here is a simple example of calculating person-time.

Each of the horizontal lines represents the person-time experienced by one person.

Note that there are five persons depicted here, subjects 1 through 5.

Each notch represents one year of completed observation.

So for example, subjects 1, 2, 3, and 5 have lines

that start at year 1, indicating that they have completed year 1.

In this depiction, an X represents death.

D represents the disease or health outcome of interest.

And L represents lost to follow-up.

A subject's person-time is the amount of time they are at risk.

So events like death or X, developing the outcome of interest or D, and lost to

follow up, L, mean that the person is

no longer at risk for the following time period.

We will add up the total person-time for the subjects in this study.

We will follow each subject's person timeline across

horizontally to count up each person's person-time contributed.

Subject 1 contributed four years of person-time before dying.

Subject 2 contributed eight years of person-time

before the end of this observation period.

Subject 3 contributed

an initial four years of person-time, then had a gap when they were not under

observation, and then contributed one more year

of person-time before getting the disease under study.

So all in all, subject 3 contributes five person years.

Subject 4 contributed five years of person-time.

Subject 5 contributed six years of

person-time before becoming lost to follow-up.

If we sum all of this person-time, we get 28 total person years.

We can now find the rate over this eight year time period.

Since only one subject developed the disease under study,

our rate is one case per 28 person years.

We often rewrite a rate to refer to more

standard number of person years, such as 100 or

1000 person years.

So the rate of one case per 28 person

years is equivalent to 3.5 cases per 100 person years.

Note that since this graph is on a scale of years, we

can very easily calculate the amount of person years contributed by each subject.

Now we'll give you the opportunity to calculate person-time in this video quiz.

Now that you understand how to calculate person-time,

let's use this information to calculate a rate.

Remember, in order to calculate a rate, we

must define our steady population, determine the number

of new cases of the health outcome, and

specify our denominator, which is person-time at risk.

Let's look at this example of calculating the rate of viral

infection among women undergoing cancer

treatment at several large medical centers.

We have 5,031 female cancer patients.

Among these women, they contributed 128,557 person-days

of observation. Among the group, 609 patients

developed a viral infection while in the hospital or within 48 hours of discharge.

So let's answer the question, what is

the rate of viral infection among this population?

Let's start with the numerator.

This is 609. The denominator

is 128,557 person-days. When

we do the division, the rate equals 0.0047.

We can then convert this to more easily interpretable

statistic to get 4.7 cases per 1000 person-days.

Now let's give you the opportunity to calculate a rate.

This concludes the segment on the measure

of health outcome occurrence known as rates.

In this segment we have learned how to define and calculate rates.

We've also defined the concept of person-time.

And you learned how to interpret rates

within the context of public health research.

[MUSIC]

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