Intended for healthcare professionals

Rapid response to:


Turning a blind eye: the success of blinding reported in a random sample of randomised, placebo controlled trials

BMJ 2004; 328 doi: (Published 19 February 2004) Cite this as: BMJ 2004;328:432

Rapid Response:

Failure to assess blinding can make clinical trials hard to interpret

Fergusson et al bring up an excellent point about the need to
formally assess blinding in clinical trials. Blinding is a key component
of a clinical trial and, along with randomization, is responsible for the
lower susceptibility of a clinical trial to bias when compared to an
observational study. When blinding is not properly accounted for, there
is the risk of methodological problems compromising the results. This
issue was recently discussed by Garbe and Suissa who noted the possibility
of detection bias in the Women’s Health Initiative (WHI) clinical trial

WHI is an excellent case where concerns with the effectiveness of the
blinding can lead to confusion about the results. There was a partial
report in the study on the high rate of unblinding (25% of subjects were
unblinded to a clinic gynaecologist) (2). Of special concern was that the
unblinding was differential (40% of women in the treatment group vs. 7% in
the placebo group) (2). This differential unblinding experienced by
gynaecologists creates concerns that study subjects or investigators could
have also experienced differential unblinding (1). However, without a
formal assessment of the blinding status of participants and investigators
it is impossible to assess the magnitude of this bias.

Properly assessing the effectiveness of blinding is an important step
forward in being able to better interpret the results of clinical trials.
Adopting the recommendations of Fergussson et al would be a solid step
forward in improving research using clinical trials.

Andrei SP Brennan, MA,
Medical Research Assistant

JAC Delaney, MSc, MA,

Caroline Hebert, MSc,
PhD Candidate (Epidemiology)

1. Garbe E and Suissa S. Issues to debate on the Women’s Health
Initiative (WHI) study. Human Reproduction 2004; 19(1): 8-13.

2. Writing Group for the Women's Health Initiative Investigators.
Risks and benefits of estrogen plus progestin in healthy postmenopausal
women: principal results from the Women's Health Initiative randomized
controlled trial. JAMA 2002;288: 321-33.

Competing interests:
None declared

Competing interests: No competing interests

23 March 2004
Andrei SP Brennan
Medical Research Assistant
JAC Delaney and Caroline Hebert
Royal Victoria Hospital, Clincial Epidemiology, R3.35, 87 Pine Ave W, Montreal, QC, Canada, H3A-1A1