Section 5: Measuring Effects and Quantifying Evidence in Biomedical Research


This section introduces commonly used measures of frequency and effect in clinical studies, and their calculation, interpretation, appropriate use, and relative merits. Results of clinical trials and other clinical studies are often reported in terms of the benefit of therapy in the treated group(s), relative to the experience of a control group which receives usual care or no care/placebo. Ability to select, calculate, and interpret measures of effect is important in reporting your own work honestly and convincingly. Ability to interpret common measures of effect, along with estimates of variation around those estimates, in the work of others is crucial in deciding whether reported findings are likely to be valid and important. This section will review and contrast the variety of basic experimental and non-experimental study designs used in clinical research. The appropriate use of each type of design depends on the state-of-the-art in the particular branch of research, the availability of appropriate study participants or data, the time and resources available, and the nature of the question to be answered. Understanding the relative merits of clinical research designs can both assist you with developing your own research program as well as aid you in judging the relative merits of studies in the biomedical literature. Similarly, Understanding and identifying potential sources of bias can aid both your own work and the interpretation of others work. This section also covers measures of change, as investigators frequently summarize change from baseline or from control using the difference or percent change without using objective criteria for choosing the best measure of change. We show how graphical methods can assist in choosing between differences, percent change, log ratios, and other measures so that the resulting measure is as independent of baseline as possible.


Learning Objectives

  1. To calculate and interpret common measures of frequency used in clinical research, describe their relative merits, and know when they are appropriately used.
  2. To calculate and interpret common measures of effect used in medical studies, describe their relative merits, and know when they are appropriately used.
  3. To interpret commonly reported confidence intervals associated with estimates of effect.
  4. To understand the relative merits and appropriate use of experimental and non-experimental research designs, and the strength of evidence from each.
  5. Understand common sources of potential bias in biomedical research, and common techniques for their minimization.
  6. To learn what constitutes a good measure of change.

Required Reading

  • K&S Chapters 14-17
  • JAMA Users Guides series published since 1993. For example:
    1. Levine M, Walter S, Lee H, Haines T, Holbrook A, Moyer V. Users guides to the medical literature. IV. How to use an article about harm. JAMA 1994; 271: 1615-9
    2. Guyatt GH, Sackett DL, Cook DJ. Users guides to the medical literature.11. How to use an article about therapy or prevention.A. Are the results of the study valid? JAMA 1993; 270: 2598-601

Recommended Reading

  • Kaiser, Stat in Med 8:1183; 1989
Topic revision: r3 - 17 Nov 2004, JonathanSchildcrout

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