If one considers a hypothetical RCT, say a total sample size of 300, separated into three strata based on surgical procedure (open surgery, closed surgery, or combined procedure). Blocking in groups of 2 will be done within each stratum.
When generating the randomization sequence, obviously one cannot know ahead of time how many patients will be randomized into each stratum.
So, how many randomization envelopes should be made for each stratum? Based on previous data, one would expect about 33/33/33 for each group, but if only 100 envelopes are made for each group, this provides no flexibility in the case that the historical data is wrong.
- play it safe and do 300 per stratum (and have a lot left over after the trial is done), or
- guess the required number based on previous data, with a buffering factor built-in?
Any tips would be appreciated!