Randomization is assigning participants to the arms of a trial by chance rather than by choice. It makes the groups comparable at the start, so a difference in outcomes can be attributed to the treatment.
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Randomization
noun · clinical research
The process of assigning trial participants to treatment arms using a chance mechanism, to prevent selection bias and balance known and unknown factors between groups.
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In short
Explained
If doctors or participants choose the treatment, the groups differ in ways that affect the outcome: healthier people may pick the new treatment, or clinicians may steer sicker patients to one arm. Any difference in results could then come from those choices rather than from the treatment. Randomization breaks the link between who the participant is and which arm they end up in. With enough participants, known and unknown prognostic factors are balanced across arms, which is why randomized trials sit at the top of the evidence hierarchy for treatment effects.
Randomization also supports the statistics. The tests used to compare groups assume that chance alone produced the allocation. And it is the basis for the intention-to-treat principle, in which participants are analysed in the arms they were randomized to. How the allocation was generated and concealed is a standard item to report; the CONSORT flow diagram generator and the SPIRIT checklist generator help cover it.
Which method suits a trial depends on its size, number of sites and the factors that must stay balanced. The table below compares the four most common.
Concealment protects the moment of enrolment: the person recruiting cannot know or predict the next assignment, so cannot steer participants. Blinding protects the time afterwards: participants, clinicians or assessors do not know the arm, so expectations do not influence care or outcome assessment. A trial can conceal allocation but still be open-label, for example surgery versus medication. A blinded study needs the blinded roles to be properly separated from the unblinded ones; see blinded and unblinded role management.
Methods
| Method | How it works | Strength | Watch out for |
|---|---|---|---|
| Simple | Each participant is assigned independently, like a coin toss | Unpredictable; easy to explain | Group sizes can end up unequal in small trials |
| Permuted block | Assignments are made in blocks (for example 4 or 6) so arm sizes stay balanced | Keeps arms equal throughout enrolment | Fixed small blocks in open-label trials can be predictable; mix block sizes |
| Stratified | Separate blocks for each combination of factors such as site or disease stage | Balances key prognostic factors | Too many strata dilute the benefit |
| Minimization | Next participant goes to the arm that best balances chosen factors, often with a random element | Good balance in small trials with several factors | Needs software; more complex to document |
Block sizes and strata are design choices for the statistician; the figures in examples are illustrative.
Set up arms, block size and stratification in the free sandbox and run test enrolments to see the assignments. No credit card.
Practice
The list or algorithm is prepared by someone independent of enrolment, usually the trial statistician or a randomization system, and its seed and specification are documented. At enrolment, the site confirms that the participant is eligible and has consented, then requests the assignment. The system records the assignment with the date, the user and any stratification values, and the participant cannot be re-randomized to a different arm if the first does not suit.
Most trials use software for this step because a paper envelope can be opened early or lost. In a multi-site study, central randomization through the EDC or an IRT/IWRS system keeps the allocation sequence in one place. You can generate a list with the free randomization list generator, or compare options in our guides to clinical trial randomization software and block randomization software. Randomization must follow the eligibility criteria and informed consent in sequence.
| Stratum | Block size | Arm A | Arm B | |
|---|---|---|---|---|
| Site 01, stage I-II | 4 | 2 | 2 | |
| Site 01, stage III | 4 | 2 | 2 | |
| Site 02, stage I-II | 6 | 3 | 3 | |
| Site 02, stage III | 6 | 3 | 3 |
In Capture
Capture runs randomization at enrolment, with block size and stratification, in the same platform as the eCRF and under one audit trail. Blinded roles never receive treatment-arm values, enforced by masked database views and row-level security rather than just the screen. Emergency unblinding is controlled and logged. Build and test the whole thing in the free sandbox before any real participant is randomized.
Day 28 is an Arm B only visit, so Arm A participants never see it.
Assigning participants to treatment arms by chance, so that the groups are comparable and any difference in outcome can be attributed to the treatment.
Randomization decides which arm a participant is in. Blinding hides the arm from participants, clinicians or assessors after allocation. A trial can be randomized without being blinded.
A method in which assignments are made in blocks, for example four at a time with two per arm, so the arms stay balanced as people are enrolled. Using varying block sizes makes it harder to predict the next assignment.
Randomizing separately within groups defined by important factors, such as site or disease severity, to keep those factors balanced between arms.
Keeping the next assignment hidden from the person enrolling the participant, so that the decision to enrol cannot be influenced by knowing which arm comes next.
After the participant has consented and been confirmed as eligible, and usually before the first dose or procedure of the assigned treatment.
Keep exploring
Free sandbox with every feature. Run dummy enrolments and see the assignments. No credit card.