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Logo of bmjThis ArticleThe BMJ
BMJ. 2007 April 14; 334(7597): 799.
PMCID: PMC1851996
Personal Views

Masking is better than blinding

Daniel Morris, specialist registrar,1 Scott Fraser, consultant ophthalmologist,1 and Richard Wormald, coordinating editor2

Why the term “blinding” should not be used in clinical trials

Consider this scenario. An elderly woman has agreed to be part of a clinical trial testing a new drug for age related macular degeneration. Her left eye has very poor vision, and the sight in her previously good right eye is rapidly declining. She is told that she is part of a double blind trial in which she and the doctor will be blinded to the treatment. Taking fright, she withdraws her consent and goes home, terrified that this “blinding” experiment may deprive her of what little vision she has left.

The term “blinding”—commonly used in clinical trials—is particularly inappropriate in the ophthalmological setting, not least because an outcome measure of a particular trial could indeed be blindness. What an odd situation when the word used to describe trial allocation is also used to describe one of the trial outcomes. As a medical term blindness does not really have a strict definition, but it has a much greater resonance in its sociocultural meaning. This common meaning is emotive enough outside eye care services, but within them the word is rarely used by practitioners and dreaded by patients.

Although it is open to much debate, one of the earliest trials that described masking of allocations seems to have been performed by a commission of inquiry appointed by Louis XVI in 1784 to investigate the medical claims of “animal magnetism.” The commission's goal was to assess whether the purported effects of this new healing method were the result of any real force or were illusions of the mind, and the participants were given what we would now call placebo or dummy treatments.1 2

In the 19th century the concept of concealment of allocation was developed further with the Nuremberg salt test of 1835. Annoyed by the rise in popularity of homoeopathy among the upper classes of Bavaria, the leading public health official challenged a prominent homoeopath to publicly test a C30 (10030) dilution of salt. One hundred and twenty citizens met in a local tavern. In front of everyone, 100 vials were numbered, shuffled, and split into two lots of 50. They were filled with either distilled water or the homoeopathic remedy. The coding list was sealed and the vials distributed by a commission of people unaware of their contents.

Three weeks later those who were given the vials were asked if they had perceived anything unusual, and only after this was the sealed list opened. Out of interest, the vast majority had perceived no effect whatsoever, but the experiment demonstrates a vigorous scientific method far ahead of its time. Those conducting it stressed that the crucial element of the experimental design was that anything that might enable the participants or those responsible for the trial to guess whether or not the actual medicine was given must be avoided; this important concept still stands today.3 4 5

Current use of the word “blinding” covers a variety of situations in a trial. Concealment of treatment allocation from those administering and those receiving it is obviously vital. Depending on the trial design, those assessing the treatment effects and the study statistician may also be unaware of allocation until the analysis is completed. Thus the word blinding is deeply ingrained in the language of trial design and evidence based medicine.6 To replace this word with a less emotive one requires a term that conveys the same obvious meaning of concealment of treatment. What is an alternative?

“Concealment of allocation” is a better term for the prevention of selection bias, while “masking” is better used for the prevention of performance and detection bias. There can be confusion between allocation concealment and masking—a common and easily made mistake. Masking is where neither the patient nor the investigator knows who is getting which treatment. It is used so that neither the patient nor the investigator can be influenced by their expectation of knowing whether patients are the privileged treated or disadvantaged control. There is no way that an investigator's observation of outcome can be biased, because he or she does not know who has had the magic ingredient. Allocation concealment is preventing the subversion of the randomisation process. If the sequence of allocation is known, it is possible to select who goes into which group. It is true that proper double masking will usually deal with allocation concealment; however, allocation concealment is mainly concerned with selection bias rather than detection bias. There are cases in unmasked studies where allocation can still be concealed—especially in surgical trials, when masking is impossible—but it is even more important to prevent selection bias.

If you have read this far and are not an eye care practitioner you have probably come to the conclusion that ophthalmologists are either too “politically correct” or far too precious about their patients. However, if we look at the origin of the term “blinding,” one dictionary defines it as “to deprive of perception or insight” but also as “partial or complete loss of sight.” Neither of these terms are ones that we would wish to associate with a rigorously designed trial. In contrast, the term “masking” is defined as “to cover in order to conceal, protect, or disguise.” We can see that the word masking in itself is a better description of what we intend to do in a trial—no matter what specialty is involved.

Thus we suggest that the term blinding should be avoided in trial design. If this is thought to be too drastic, we would at least ask practitioners to take care when they use it, either when describing a trial to a patient or within consent forms or patient information leaflets. Doctors are notorious for using words that patients either do not understand or that have a completely different meaning to them, and blinding is perhaps a term we can start getting used to doing without.

The word masking in itself is a better description of what we intend to do in a trial


1. Lavoisier A, Franklin B. Rapport des commissaires chargés par le roi du magnetisme animal. Paris: Imprimerie royale, 1784
2. Franklin B, Majault, Le Roy, Sallin, Bailly JS, D'Arcet, et al. Report of Dr Benjamin Franklin, and Other Commissioners, Charged by the King of France, with the Examination of Animal Magnetism, as Now Practices in Paris (trans. William Godwin). London: J Johnson, 1785
3. Löhner G, on behalf of a society of truth-loving men. Die homöopathischen Kochsalzversuche zu Nürnberg [The homeopathic salt trials in Nuremberg]. Nuremberg, 1835
4. Reuter JJ. Sendschreiben an Dr E Fr Wahrhold als Erwiederung auf dessen Schrift “Auch etwas über die Homöopathie.” Nuremberg, 1835
5. Stolberg M. Inventing the randomised double-blind trial: the Nuremberg salt test of 1835. The James Lind Library, 2006.
6. Day SJ, Altman DG. Blinding in clinical trials and other studies. BMJ 2000;321:504. [PMC free article] [PubMed]

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