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National Information Center on Health Services Research and Health Care Technology (NICHSR)

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Introduction to Health Services Research : A Self-Study Course

Module 6: Basic Components of a Study (Page 6 of 11)

Surveys | Cohort Studies | Case-Control Studies | Randomized Clinical Trials

Strengths and Pitfalls: Cohort Studies

Cohort studies observe groups of individuals before they develop a disease or a particular outcome.

Cohort studies observe groups of individuals before they develop a disease or a particular outcome. Researchers select a group with an exposure or experience and a similar group without that exposure or experience. They follow the groups over time and measure the results. Over time the group will consist of both well and ill people.

For example, researchers observe people with asbestos exposure and people without asbestos exposure over a long period of time to detect differences in their rates of lung disease.

Cohort studies tell researchers what happens to people over time.

Advantages

There are several advantages to cohort studies. They have the power to detect many different outcomes of an exposure. Because the people are all healthy when the study begins, researchers aren't likely to misclassify them based on knowledge of their outcomes. In the analysis phase, cohort studies allow researchers to calculate a relative risk of developing a disease based on different exposures.

Disadvantages

However, several disadvantages exist as well. Cohort studies cannot tell us if an experience caused a result, but they give us strong clues. For many outcomes, it may take many years to detect changes in the groups. Cancer often requires decades to develop. A decline in mortality rates due to mammography will take many years to evaluate. Researchers may be tempted to decrease the length of the study, or participants may drop out of the study as time passes. Because of the time involved and number of participants needed, cohort studies may be very costly. Cohort studies can measure many outcomes but (usually) only one exposure.

Exercise

  1. With the death of Terri Shiavo, Americans have become more concerned about end-of-life care and related issues. Have any cohort studies been done on terminal care? The linked search will retrieve citations quickly for you to examine. Are there any implications for health services research practice based on what you see in the citations that show up?

Discussion Questions

  1. Given how long it takes some diseases to develop, is it really worth the time and money necessary to implement a cohort study?
  2. Think about some of the cohort studies you have heard or read about in the course of your work. Has the resulting research been of value? Have any of the results impacted your life? Have any recent studies been stopped before the end of the study due to excess mortality or poor health outcomes? How would you search for such studies?
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