Allocation:
key points
i)
Simple randomization
Allocation
is simply by tossing a coin when each new patient appears. A good method in large trials but in smaller
trials there is a surprisingly high chance of getting notable imbalance between
the groups. A very simple imbalance is
when the groups are different sizes: a more serious one is when allocation bias
is present.
ii)
Random Permuted Blocks (RPBs)
A method for ensuring that group sizes never get too far out of balance. A potential problem with the method is that
if the block length becomes know, the method is predictable and selection bias
can arise. Randomly varying block length
can help.
iii)
Unequal randomization
Although maximum power can be obtained when the allocations to the groups are
in the ratio 1:1, the loss in power is slight if the ratio departs only
slightly from 1. There can be practical
advantages to unequal allocation, which might be worth considering in some
applications.
iv)
Stratification
If there are important prognostic factors
which, if they were distributed unequally between the treatment groups would
give rise to a serious bias, then it may be prudent to intervene in the
randomization process to ensure balance between these factors. This can be achieved by stratification, in
which RPBs are used within each stratum defined by the prognostic factors
v)
Minimization
Stratification can be cumbersome if there are too many prognostic factors:
minimization is a method which is can provide balance in a less cumbersome way.