Speaking Notes

PADM 5324

September 1, 2009

Dr. Neubauer

WHERE WE ARE

• Dr. Keith Hamon and Dr. Tom Clancy are with us this evening to help us regarding collaborative writing online.
• Are there any "In the News" reports this evening?
• I am pleased to see that we are beginning to use Google Docs to write the paper together. However, please do not "copy and paste" large blocks of text into the page. LINK to your online resources! What appears on the collaboration page should be your own writing.
• I don't have a homework one assignment ready yet. I will move the due date to Sept. 29. Please refer to course calendar on the course Web site.

CHAPTER 3 of our textbook . . .

MORBIDITY is about measuring the occurrences of disease in a population.

MORTALITY is about measuring the occurrences of death in a population.

A POPULATION includes all the people of interest, usually defined by location (geographic boundaries). It could be defined socially or organizationally, such as the undergraduate student body of a university or all persons having Medicare.

OCCURRENCE of disease can be measured using RATES or PROPORTIONS. A rate has a time dimension. A proportion is a "snapshot" of a moment in time.

An INCIDENCE RATE is usually expressed "per 1000" people in the population.

The NUMERATOR of an incidence rate is the NUMBER OF NEW CASES observed (or reported) in the population during a specified period of time.

The DENOMINATOR of an incidence rate is the total number of people in the population WHO ARE AT RISK of the disease/condition during the specified period of time.

The result of division is a fraction or decimal which is then MULTIPLIED BY 1000 to produce an incidence rate per thousand.

So, let's assume that the population is 11 students enrolled in a course. 10 of them are at risk (could contract) a particular disease. In the period of CY 2009, two of them actually get the disease. The incidence rate (projected to 1000) is as follows.

2/10 = .2 * 1000 = 200 per 1000

Things to notice:

• There is a specified period of time.
• The denominator is 10 because for some reason one student is not at risk.
• Everyone in the denominator could possibly become a "member" of the numerator.
• We were apparently able to monitor everyone one in the class for the entire year.
• It is possible that someone in the class caught the disease during the year but there was no observation or report of that case.
• This is about NEW CASES. If a student already had the disease at the beginning of the year they apparently should not be counted either in the numerator or the denominator.
• This calculation says nothing about the PREVALENCE or DURATION of instances of the disease.
• Total PERSON-TIME is the same as the number of persons at risk multiplied by the total period of observation. In other words, no one in the population dropped out or died during the period of observation.
• Apparently, another assumption is that there can be no more than one incidence of the disease in a single person for the duration of the period of time.

WHEN WE NEED TO ADJUST FOR "DROPOUTS" the measure is called INCIDENCE DENSITY. The formula is on page 40 of our textbook.

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Just to make things interesting, AN ATTACK RATE IS NOT A RATE. It is a proportion. I think it basically has to do with exposure rather than with being a member of a "population." The example in the textbook is a food-born attack rate which represents the PROPORTION of the people who became sick after eating a certain food. Notice that there is no period of time specified. What makes a person a member of the "population" is having been exposed.

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PREVALENCE is important because there are so many chronic diseases/conditions now. The numeration is not the number of NEW cases but is rather the number of all cases, either at A SPECIFIC POINT IN TIME (snapshot) or during a period of time. For a period of time, a person gets counted in the numerator if he/she has had the disease/condition at any moment during the period (or for the entire period). Please see the formula on page 43.

One important realization is that as public health efforts increase the apparent prevalence of diseases and conditions is likely to increase. This is not because public health efforts are "bad" but because additional cases are identified that would not otherwise be recorded.

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There are many potential problems with the use of surveys to collect epidemiological data. Please see page 49.

The confusion of measures of incidence with measures of prevalence can lead to some incorrect conclusions. In the example in the book, the health of the people in "Hitown" may appear worse than the health of the people in "Lotown" because in Hitown people survive with the disease/condition and in Lotown people die quickly of the same disease/condition. In other words, a high prevalence might not be a bad thing, given the alternative.

GIS systems can be useful in identifying patterns of incidence in geographic areas and for gaining insights into possible sources and VECTORS.