Speaking Notes

PADM 5324

October 6, 2009

Dr. Neubauer

We are now entering the second section of our textbook which has an emphasis on using epidemiology to identify the causes of diseases. This section has eight chapters.

By the course calendar, we are behind, but that is okay. Let's cover chapter 9 and a little preview of chapter 10 this evening.

Chapters 9 and 10 cover two ways to design a study to try to answer questions regarding CAUSATION of diseases and conditions. One method is called a COHORT STUDY and the other is called a CASE-CONTROL study. The explanation of the two of them is on page 179. It is a bit confusing. I think the bottom line is the following (on page 179):

"What distinguishes the two study designs is whether the study begins with diseased and nondiseased people (case-control study) or with exposed and nonexposed people (chort study)."

In either case, the finding is based upon a comparison of two groups -- one of which is a CONTROL GROUP in either case. It is tempting to describe the difference as "going forward" or, "going backward," although the textbook says that is not the way to think about it.

IN EITHER CASE, IT IS THE SELECTION OF THE CONTROL GROUP THAT IS CRITICALLY IMPORTANT. The study by Pearl was flawed because the controls came from a pool that was heavily weighted with tuberculosis patients. See page 181.

MY OWN CONTRIBUTIONS . . .

• In medical science there is rarely a one-to-one correspondence between cause and effect.
• For example, smoking causes lung cancer but that does not say that everyone who ever smoked in his/her life will get lung cancer. On the other hand, some people who have never smoked get lung cancer.
• It is probably more accurate to say that some things/events strongly contribute to or are highly correlated with particular diseases/conditions.
• The cause and the effect are often not proximate in time. The beginning of a disease is often difficult to pin down to a particular day or event.
• Everyone is exposed to a huge number of things that "cause" various conditions/diseases. Some get "it" and others don't and it may well be largely luck that determines who does and who does not.
• Most conditions/diseases do not have a single cause.
• All COMPLEX SYSTEMS are by definition difficult to describe and understand. Human bodies are very complex. Human environments are very complex.
• People are different. Individual genetics is important and we don't yet have the tools to tap into that completely.
• The mind-body connection is not fully understood.

The control group above are those who don't have the disease. If X causes the disease we would expect a larger percent of those who have the disease to have been exposed to X than the comparable percent in the control group.

The control group above is the group who were not exposed to X. If X causes the disease we would expect a larger percent of those who were exposed to X to have the disease than the percent of those not exposed to X who have the disease.

1. Does it appear from the results of the study above that exposure to X causes the disease? If yes, why?

2. Does it appear from the results of the study above that exposure to X causes the disease? If yes, why?

CHAPTER 9 -- Cohort Studies

These things make it difficult to nail down EXACTLY WHAT CAUSES A PARTICULAR PERSON TO HAVE A PARTICULAR CONDITION/DISEASE.

Nevertheless, GENERAL PATTERNS OF CAUSATION are possible to identify and are valuable public health knowledge resources.

THE LONG FORM OF A COHORT STUDY

• Find a large population sample of people who don't have the disease of interest.
• Identify them and ask their permission to follow up with them for many years to come.
• Collect data from each of them on a regular basis, including data about exposures to whatever list of things (possible causes) interest the researchers.
• In time, for a PARTICULAR POSSIBLE CAUSE identify those who have been exposed to it and those who have not be exposed.
• Continue monitoring the members of the cohort, for many years if necessary.
• Among those who have BEEN EXPOSED to WHATEVER take note of who develops the disease of interest and who does not.
• Among those who have NOT BEEN EXPOSED to WHATEVER take note of who develops the disease of interest and who does not.
• If a substantially larger percentage of those EXPOSED to WHATEVER get the DISEASE OF INTEREST then WHATEVER apparently "causes" the disease.

BENEFITS OF THE LONG METHOD DESCRIBED ABOVE

• The research is relatively NOT INTRUSIVE.
• There was no need to assign people to groups.
• It is probably possible to study MANY DIFFERENT HYPOTHESES all at the same time.

PROBLEMS WITH THE LONG METHOD DESCRIBED ABOVE

• It may take many, many years.
• Medical knowledge is likely to change in the mean time and there is no going back to gather data that was not originally anticipated to be needed.
• Some people may drop out of the cohort (by dying, for example).
• People may be affected by the fact that they know they are part of a study.