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The process used in epidemiological studies and clinical trials in which the participants, investigators and/or assessors remain ignorant concerning the treatments which participants are receiving. The aim is to minimise observer bias, in which the assessor, the person making a measurement, have a prior interest or belief that one treatment is better than another, and therefore scores one better than another just because of that.

In a single blind study it is may be the participants who are are blind to their allocations, or those who are making measurements of interest, the assessors.

In a double blind study, at a minimum both participants and assessors are blind to their allocations.

In some circumstances much more complicated designs can be used, where blinding is described at different levels.

To achieve a double-blind state, it is usual to use matching treatment and control treatments. For instance, the tablets can be made to look the same, or if one treatment uses a single pill once a day, but the other uses three pills at various times, all patients will have to take pills during the day to maintain blinding.

If treatments are radically different (tablets compared with injection), a double-dummy technique may be used where all patients receive both an injection and a tablet, in order to maintain blinding.

The important thing to remember is that lack of blinding is a potent source of bias, and open studies or single-blind studies are potential problems for interpreting results of trials.

Concealment of allocation

The process used to prevent foreknowledge of group assignment in a randomised controlled trial, which should be seen as distinct from blinding. The allocation process should be impervious to any influence by the individual making the allocation by having the randomisation process administered by someone who is not responsible for recruiting participants; for example, a hospital pharmacy, or a central office. Using methods of assignment such as date of birth and case record numbers (see quasi random allocation) are open to manipulation.

Adequate methods of allocation concealment include: centralized randomisation schemes; randomisation schemes controlled by a pharmacy; numbered or coded containers in which capsules from identical-looking, numbered bottles are administered sequentially; on-site computer systems, where allocations are in a locked unreadable file; and sequentially numbered opaque, sealed envelopes.