Interpreting Epidemiologic Reports and Contraceptive Eligibility Criteria
Interpreting Epidemiologic Reports and Contraceptive Eligibility Criteria
Clinical practice is the ultimate distillate of evidence, judgment, and experience. The safety, side effects, and benefits of treatments are established by epidemiologic studies. The clinician must determine whether the data derived from epidemiologic studies are clinically relevant and useful. The incorporation of the data into clinical practice depends upon that determination. In this chapter, we provide a guide for interpreting epidemiologic reports, a guide intended to help clinicians make appropriate determinations regarding epidemiologic data, and ultimately to apply this information properly in clinical practice.
The Hierarchy (in Descending Order) of Epidemiologic Studies
Randomized Controlled Trials
A randomized trial is a true clinical experiment in which an intervention is compared with a standard treatment, no treatment, or a placebo, with allocation to treatment by chance. More than one comparison can be made within a study. Participants theoretically have a random (an equal and unbiased) chance of being assigned to each group in the study, and the participant characteristics should be nearly if not totally the same in each group. In crossover trials, participants are randomly assigned to one treatment group and later switched to the other group, and thus the participants serve as their own controls.
Advantages: |
Provides scientific, epidemiologic proof. |
Disadvantages: |
Very expensive and time-consuming. Only a limited number of hypotheses can be evaluated in any one study. |
Example: |
The Women’s Health Initiative |
Observational Studies (Nonexperimental Studies: Observation Without Intervention)
Cohort studies: A prospective follow-up over a long period of time of a large group of individuals, also referred to as longitudinal or follow-up studies.
Exposure information is collected from all subjects who are disease free, and subjects are followed over time to determine who develops disease. A historical cohort study is retrospective, following a cohort in past time, not from current time onward.
Advantages: |
A relatively accurate estimation because of large numbers, can evaluate changes over time, avoids recall bias. |
Disadvantages: |
Expensive, lengthy in time, and subject to biases (particularly selection bias and surveillance bias) making the two groups being compared unequal. |
Example: |
The Nurses’ Health Study |
Case-control studies: A retrospective comparison of a group of individuals with a condition or problem compared with a carefully selected control group. Subjects are selected according to specific inclusion and exclusion criteria. The exposure history of those with disease and those with no disease is collected and compared.
Advantages: |
Relatively quick and inexpensive because of small sample sizes. |
Disadvantages: |
Subject to biases and errors. |
Example: |
World Health Organization (WHO) Collaborative Study of Cardiovascular Disease and Steroid Hormone Contraception |
Cross-sectional studies: A description of a group of individuals at one point in time.
Advantages: |
A reliable method to estimate prevalence; quick and inexpensive. |
Disadvantages: |
Cannot assess changes over time and very susceptible to sampling error (the group is not representative of the actual population of interest). |
Example: |
The Health and Nutritional Examination Survey |
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