Editing Randomized controlled trial (section)

==== Restricted ====
To balance group sizes in smaller RCTs, some form of [[restricted randomization|"restricted" randomization]] is recommended.<ref name="Lachin-1988b" />  The major types of restricted randomization used in RCTs are:
* [[Randomized block design|Permuted-block randomization]] or blocked randomization: a "block size" and "allocation ratio" (number of subjects in one group versus the other group) are specified, and subjects are allocated randomly within each block.<ref name="Schulz-2002" /> For example, a block size of 6 and an allocation ratio of 2:1 would lead to random assignment of 4 subjects to one group and 2 to the other. This type of randomization can be combined with "[[stratified randomization]]", for example by center in a [[multicenter trial]], to "ensure good balance of participant characteristics in each group."<ref name="Moher-2010" /> A special case of permuted-block randomization is ''random allocation'', in which the entire sample is treated as one block.<ref name="Schulz-2002" /> The major disadvantage of permuted-block randomization is that even if the block sizes are large and randomly varied, the procedure can lead to selection bias.<ref name="Lachin-1988a" /> Another disadvantage is that "proper" analysis of data from permuted-block-randomized RCTs requires stratification by blocks.<ref name="Lachin-1988b" />
* Adaptive biased-coin randomization methods (of which urn randomization is the most widely known type): In these relatively uncommon methods, the probability of being assigned to a group decreases if the group is overrepresented and increases if the group is underrepresented.<ref name="Schulz-2002" /> The methods are thought to be less affected by selection bias than permuted-block randomization.<ref name="Lachin-1988b" />