confounding and the language of experimentation part ii - importance of proper comparisons and...
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Confounding and the Language of ExperimentationPart II - Importance of Proper
Comparisons and Randomization
This video is designed to accompany
pages 13-18
in
Making Sense of UncertaintyActivities for Teaching Statistical
ReasoningVan-Griner Publishing Company
Arthroscopic Knee Surgery
Traditional Design
Patients come to doctor with knee symptoms. Range of motion measured
Arthroscopic surgery used to treat the condition
Patients asked if they felt better and range of motion re-measured.
Largely:• subjects felt better • range of motion was
better
What’s Missing
No Placebo ComparisonHow do we know that the results were not confounded by the placebo effect?
Seriously!
Source: N Engl J Med. 2002 Jul 11;347(2):81-8.
Title: A controlled trial of arthroscopic surgery for osteoarthritis of the knee.
Authors: Moseley JB, O'Malley K, Petersen NJ, Menke TJ, Brody BA, Kuykendall DH, Hollingsworth JC, Ashton CM, Wray NP.
Place: Houston Veterans Affairs Medical Center, Baylor College of Medicine, Houston, TX 77030, USA.
Controlled Study w/ Placebo
METHODS:
A total of 180 patients with osteoarthritis of the knee were randomly assigned to receive arthroscopic débridement, arthroscopic lavage, or placebo surgery.
Patients in the placebo group received skin incisions and underwent a simulated débridement without insertion of the arthroscope. Patients and assessors of outcome were blinded to the treatment-group assignment.
Outcomes were assessed at multiple points over a 24-month period with the use of five self-reported scores--three on scales for pain and two on scales for function--and one objective test of walking and stair climbing. A total of 165 patients completed the trial.
Surprising Conclusion
CONCLUSIONS:
In this controlled trial involving patients with osteoarthritis of the knee, the outcomes after arthroscopic lavage or arthroscopic débridement were no better than those after a placebo procedure.
(N Engl J Med 2002;347:81-8.)Copyright © 2002 Massachusetts Medical Society.
Language
• Response Variables – Level of pain and function
• Explanatory Variable – Type of Procedure (Real or Placebo)
• Subjects – The participating patients with osteoarthritis of the knee
• Confounding– In the traditional design, the placebo effect created the confounding and compromised the inference.
Diagraming the Faulty Design
Compare
Measure Pain and Function
Measure Pain and Function
Before or No Surgery Surgery
Placebo Respons
e
Diagraming the Careful Design
Compare
Measure Pain and Function
Measure Pain and Function
Placebo Surgery Real
Surgery
Importance of Randomization
USE OF AN AIDS VIRUS DRUG CURBED ON THE BASIS OF DATABy ANDREW POLLACK, Special to the New York TimesPublished: April 16, 1987
SAN FRANCISCO, April 15— The Food and Drug Administration has told a California pharmaceutical company that there is not yet enough evidence to justify more widespread use of its antiviral drug ribavirin to treat patients with early symptoms of infection with the AIDS virus.
Importance of Randomization
Reported Results
Treatment Number of Patients
How Many Developed Aids
800 mg 52 0
600 mg 55 6
Placebo 56 10
Looks Convincing!
FDA Looked Closely …
There was evidence that the sickest patients had been assigned to the placebo group.
And the healthiest patients to the Ribavirin treatments.
Inference of “effective treatment” was compromised.
Language
• Response Variable – Development of AIDS (yes/no)
• Explanatory Variable – Type of Intervention (Ribavirin level, Placebo)
• Subjects – The participating patients with pre-AIDS symptoms
• Confounding– Lack of randomization put healthiest patients in the Ribavirin treatment groups.
Benefits of Randomization
Helps to keep the comparison groups – treatment groups – as much alike as possible.
In that sense, randomization addresses confounding in a very direct way.
One-Sentence Reflection
The placebo effect and lack of randomization can create very real obstacles to making credible inferences from experimental data.
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