Research brief · 2026-09-09
What Makes a Control Group Good — or Useless?
A control group exists to answer one question: compared to what would have happened anyway, did the treatment do anything? Several things make a person report feeling better that have nothing to do with a treatment's biology. Natural healing. Regression to the mean, since people often enroll when symptoms are at their worst. Extra attention. Plain expectation. A control that is not blinded or not convincing cannot separate any of that from a real effect, and the failure is quiet, because the trial still produces a number that can look positive.
The clearest recent example is a 2021 trial in JAMA. Kearney and colleagues randomized 240 adults with chronic Achilles tendinopathy to a real platelet-rich plasma injection into the tendon or a sham: a needle inserted under the skin, never advanced into the tendon. Patients did not know which they received. At six months, tendon-function scores were 54.4 for the real injection and 53.4 for the sham, an adjusted difference of minus 2.7 points on a 100-point scale, against the 12 points the trial had set as clinically meaningful. An earlier 2015 Cochrane review of 18 smaller trials had been murkier.
The logic works outside injections. In 2002, Moseley and colleagues in the New England Journal of Medicine randomized 180 patients with knee osteoarthritis to arthroscopic surgery or placebo surgery with skin incisions only. Both patients and assessors were blinded. Across two years, there was no meaningful difference between placebo and either real surgery. A 2017 analysis in the Orthopaedic Journal of Sports Medicine found knee arthroscopies for osteoarthritis fell from 10.6 to 7.2 percent of cases after publication.
A good control is convincing, blinds the people scoring outcomes, has enough patients to rule out a moderate placebo response, and sets its primary outcome in advance. What neither trial proves is that these treatments have zero biological activity. They show that whatever exists did not beat a convincing sham at that size. Decisions about any procedure belong with a physician. Sources are listed below.
Sources
- Kearney RS et al., Effect of platelet-rich plasma injection vs sham injection on tendon dysfunction in chronic midportion Achilles tendinopathy (n=240)
- JAMA, 2021
- Moseley JB et al., A controlled trial of arthroscopic surgery for osteoarthritis of the knee (n=180)
- New England Journal of Medicine, 2002
- Kearney RS et al., Injection therapies for Achilles tendinopathy (18 trials, n=732)
- Cochrane Database of Systematic Reviews, 2015
- Amin NH et al., Changes within clinical practice after a randomized controlled trial of knee arthroscopy for osteoarthritis
- Orthopaedic Journal of Sports Medicine, 2017
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