Stats
What is block randomization (June 30, 2006)
Category: Definitions, Category: Randomized trials
Several people have asked me about block randomization, and while I discuss it briefly on
one of my web pages,
there is a better definition on the Consort
Statement web site that defines several variations of randomization including block
randomization.
Blocking can be used to ensure close balance of the numbers in each group at any
time during the trial. After a block of every 10 participants, for example, five would
be allocated to each arm of the trial (105). Improved balance comes at the cost of
reducing the unpredictability of the sequence. Although the order of interventions
varies randomly within each block, a person running the trial could deduce some of the
next treatment allocations if they discovered the block size (106). Blinding the
interventions, larger block sizes, and randomly varying the block size can ameliorate
this problem.
www.consort-statement.org/examples8b.htm
It is worth noting that a process that some people use, creating a separate randomization
list for different centers or for different demographic groups in order to assure good
balance across these important factor, is actually stratified randomization and not block
randomization.
07/08/2008.
