A key feature of a randomized control trial is the process of randomization. Through randomization, we allocate interventions to trial arms in a way that ensures that neither the investigators nor the participants know or can predict ahead of time which treatment a subject will receive. Proper randomization procedures and reporting include the following steps.
Generation of the random allocation sequence, including details of any restrictions.
Implementation of the random allocation sequence: information on who generated the allocation sequence, who enrolled the participants, and who assigned them to their groups.
In this article and the next one, I will discuss popular methods of generating the random allocation sequence. I will explain allocation concealment and implementation of randomization in a separate article.
Many methods for assigning subjects to groups have been used, including sequential treatment assignment, or assignment according to an unrelated factor, such as patient file number, day of the week or month, birthday, or participants’ initials. However, these methods are not truly random. They are open to manipulation and have been termed “quasi-randomization” methods.
Popular methods that deliver true randomized allocation include simple, restricted or block, and stratified techniques.
Simple randomization generates randomization lists according to random tables or appropriate software; this resembles the toss of a coin. Random tables include sequences of numbers that occur randomly, with no discernible pattern and with similar frequency, from which we can select numbers in any direction and from any starting point. The Table provides random number sequences from 0 to 9 and can be used as follows: for a 2-arm trial, numbers 0 to 4 can be assigned to treatment A, and numbers 5 to 9 to treatment B. An allocation sequence using this Table and going vertically for 58 patients would be AABBAAAAABBBAAAABBBBBBBBAAAABBBAAAABABAAAABABBAAABAAAABABB ( Table columns 1 and 2).