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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 5: Quality Filtering and Evidence-Based Medicine and Health (page 7 of 15)
Introduction | Sampling | Assignment | Assessment | Analysis | Interpretation | Extrapolation

Assignment (grouping)

To evaluate a specific treatment, preventive service, or health intervention, researchers assign the people in their sample into groups. Sometimes, they choose ready made groups, such as smokers and non-smokers and observe differences in their health due to their smoking status.

In other, more experimental studies, investigators actually divide people into groups and expose them to different experiences, such as drugs, cancer screening exams, or diets.

In either type of design, the people in each group must be alike except for the factor being tested. If they also differ on factors that influence the study results, the study will not be accurate. These factors are known as confounders because they confound or mislead our interpretation of the outcome of the study.

MRFIT Study

A famous example of this problem is the MRFIT (Multiple Risk Factor Intervention Trial) study. In this study of heart disease, one group was encouraged to adopt lifestyle changes that would lower its heart disease risk. However, the control group to which the first group was compared voluntarily adopted many of the same lifestyles that also lowered its risk. Therefore, it appeared that there were no differences between the groups and that the behavior changes had no effect on heart disease risk.

Choice of Subjects in a Study

Choosing people for comparison is a crucial feature in study design. When reading the literature, examine the table that describes the people in each group. Check to see that they are similar in age, socioeconomic status, health characteristics, and others (excluding the factors being studied). If it appears that they are very different AND that these differences could affect the study's results, question the conclusions of the study.

In a clinical trial study on watchful waiting for surgery for benign prostatic hyperplasia (BPH), the authors describe the randomization process which increases the likelihood that the men would have an equal chance of being assigned to each treatment arm.

Eligible patients were randomly assigned to transurethral resection or watchful waiting... The characteristics of the two groups were similar (Table 1). The mean (+-SD) age of all randomized patients was 66 +- 5 years. ... (Wasson et al., 1995).

Discussion Questions

  1. How important is it for you to examine the table in the Methods section of an article that describes the people in each group in a survey? What kinds of information about the study groups do you expect to find in that table?
  2. The key recommendation above is that librarians should examine the table that describes the people in each group when they are reading the literature. Discuss why it is important for quality filtering.
  3. The following information was given about a study on watchful waiting for surgery for benign prostatic hyperplasia (BPH): Eligible patients were randomly assigned to transurethral resection or watchful waiting... The characteristics of the two groups were similar (Table 1). The mean (+-SD) age of all randomized patients was 66 +- 5 years. ... (Wasson et al., 1995). Discuss why it is important to know the age and whether or not the two groups were similar. What other group characteristics might you look for in such a study? Why?
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