Question 29

With reference to a randomized controlled trial, briefly describe the terms “blinding” and “allocation concealment”.

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College Answer

•    Blinding  and  allocation  concealment  are methods  used to reduce  bias in clinical trials.
•    Blinding: a process by which trial participants and their relatives, care-givers,  data collectors and those adjudicating outcomes are unaware of which treatment is being given to the individual participants.

-      Prevents   clinicians   from   consciously   or   subconsciously   treating   patients differently based on treatment allocation
-      Prevents  data  collectors  from  introducing  bias  when  there  is  a  subjective assessment to be made for eg “pain score”
-      Prevents outcome  assessors  from introducing  bias when there is a subjective outcome assessment to be made for eg Glasgow outcome score.

•    Traditionally, blinded RCTs have been classified as "single-blind," "double-blind," or "triple-blind";  The  2010  CONSORT  Statement  specifies  that  authors  and  editors should not use the terms "single-blind," "double-blind," and "triple-blind"; instead, reports of blinded RCT should discuss "If done, who was blinded after assignment to interventions  (for example,  participants,  care providers,  those assessing outcomes) and how.

Allocation concealment is an important component of the randomization process and refers to  the  concealment   of  the  allocation  of  the  randomization   sequence  from  both  the investigators  and  the  patient.      Poor  allocation  concealment  may  potential  exaggerate treatment effects.
Methods used for allocation concealment include sealed envelope technique, telephone or web based randomization.

Allocation concealment effectively ensures that the treatment to be allocated is not known before the patient is entered into the study. Blinding ensures that the patient / physician is blinded to the treatment allocation after enrollment into the study.


The question is a 10-mark question, but it for some reason asks for one to "briefly describe" these concepts. Judging from the college answer, a truly brief description was not the expected response.

LITFL has a thorough summary, which is not brief.

If one were to briefly describe these concepts, one would produce something like this:

Allocation concealment:

  • Ensures that the patients and investigators cannot predict which treatment will be allocated to which patient before they are enrolled in the study.
  • Prevents selection bias


  • Ensures that the patients and investigators remain unaware of which treatment is being administered to which idividual patient.
  • Prevents detection bias and observer bias

And if one were to go to town on this topic, one would produce something like this:

Allocation concealment

  • Allocation concealment is the technique of reducing bias by preventing the prediction of treatment allocation before the allocation is completed.
  • Thus, both the investigators and the patients cannot predict which patient will be selected for the treatment group and which patient will be selected for the control or placebo group
  • This ensures that no selection bias influences the treatment allocation; i.e. patients are allocated randomly, which avoids the problem of investigators choosing most suitable candidates for the treatment group.
  • Allocation concelament can be performed either with sealed envelopes, web-based randomisation, or random number generation.
  • Alternative techniques also exist; for instance one can randomise patients according to the day of the week of their presentationto hospital, or according to the odd or even date of the calendar month. The allocation is not concealed, but the investigators still have little control over the allocation.


  • Blinding is the technique of reducing bias by concealing the allocation of the treatment and control groups from either the patients, the investigators, the statistical analysts, or everyone involved.
  • Reduces bias by preventing anybody from knowing which patient is receiving which treatment, and thus decreasing the likelihood that a particular group will receive preferential treatment, that a particular group will be assessed differently, or that a particular group will develop expectations of their treatment,
  • Reduces detection bias by blinding the investigators
  • Reduces observer bias by blinding the observers
  • Reduces recall bias by blinding the patients
  • The exact method of the blinding should be transparently reported (as per he CONSORT statement). Thus, the reader of the article should be able to immediately discern who was blinded and how.


Schulz, Kenneth F., and David A. Grimes. "Allocation concealment in randomised trials: defending against deciphering." The Lancet 359.9306 (2002): 614-618.

Forder, Peta M., Val J. Gebski, and Anthony C. Keech. "Allocation concealment and blinding: when ignorance is bliss." Med J Aust 182.2 (2005): 87-9.

Schulz, Kenneth F. "Assessing allocation concealment and blinding in randomised controlled trials: why bother?." Evidence Based Mental Health 3.1 (2000): 4-5.