Turn an Allocation Ratio into Transparent Group Counts

Name the groups before applying a ratio

Write the group names in the same order everywhere the allocation ratio appears. A ratio of 2:1 means little if the protocol, recruitment spreadsheet, and randomization system disagree about which group receives the larger share. Record the unit being allocated, such as a participant or clinic, separately from the number of measurements collected.

The SPIRIT 2025 trial-design explanation calls for a clear description of the design, allocation ratio, and study framework. Group-count arithmetic supports that description, but does not justify the overall sample size or a particular allocation choice.

Convert the ratio into planned counts

First reduce the positive integer ratio to its smallest terms: 4:2 becomes 2:1. An exact 2:1 ratio requires complete units of three allocations. The tool treats the entered total as a minimum and rounds upward to a multiple of the sum of the reduced ratio components. It does not independently round proportional group counts.

Use the allocation counts tool to explore the arithmetic. Record the entered total, ordered ratio, output counts, and rounding convention in the allocation planning worksheet. These calculations describe a target distribution, not a generated random sequence.

Work through a fictional target

Suppose a fictional study sets a minimum target of 180 participants with intervention-to-control allocation of 2:1. The counts are 120 and 60. If the minimum is instead 181, the next exact-ratio total is 183, giving 122 and 61. The tool reports the two additional allocations explicitly rather than returning 121 and 60 with an altered ratio.

For the tool's default minimum of 101 and ratio 2:1, the result is 102 overall, with 68 and 34 per arm. These fictional targets illustrate arithmetic only. Review the higher total against the approved recruitment plan; the smallest ratio unit is not a recommendation for a randomization block size.

Connect counts to the broader plan

Clarify whether N refers to randomized, enrolled, exposed, or analyzable participants. If allowing for loss to follow-up, state the assumptions and whether they differ between arms. Changing the allocation ratio can change precision and sample-size requirements; the count tool does not calculate power or establish adequate precision.

Separate intended proportions from how assignment is implemented. Simple randomization can produce realized imbalance, while restricted procedures have their own balance rules and operational requirements. A list with exactly the target counts is not proof of unpredictable, concealed assignment.

Finish with an agreed group-order convention, count table, rounding policy, and cross-reference to the sample-size and randomization specifications. Check all three together whenever recruitment targets change. Document the rationale for any unequal allocation and avoid choosing a ratio solely because it produces an appealing number of participants in one group.

Source checked on 1 November 2026; fictional allocation arithmetic independently recalculated.