= Solution
Use individual <stratified randomization> within each joint combination of prison and the three prespecified addiction-management categories. Within every joint stratum, use <permuted-block randomization> with equal numbers of active and control assignments in each completed block. This directly balances allocations within prison and within management category, rather than hoping that simple independent allocation will achieve both.
Run the allocation sequentially, since the availability and frequency of management categories change during recruitment. Completed blocks balance contemporaneously, so the expanding category is not assigned predominantly to one trial arm. Use varying block sizes and centrally protected <allocation concealment> to prevent prediction of future assignments. Incomplete final blocks allow a small residual imbalance, and an odd stratum total cannot be split exactly equally. If many joint strata are very sparse, a prespecified <randomized minimization in clinical trials> scheme can instead target the prison and management margins, retaining a random component. The analysis should account for the important design strata.
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