Enrolment and randomisation¶
This chapter explains how the portal decides which arm a participant goes into, and how it knows when to pause or stop recruiting. You set the inputs in Step 3 · Treatment Phases; this is what the portal does with them.
The two things you set¶
- Per arm, participants to complete
For each active or sham arm, the number of participants you expect to complete it.
- Per trial, estimated completion rate
The fraction of enrolled participants you expect to finish (default 0.5).
Everything else is worked out from these.
What the portal derives¶
- Total completion target
The sum of the per-arm targets. This is the number of completed participants the trial is aiming for.
- Randomisation shares
Each arm’s target divided by the total. With targets 50 / 25 / 25 the shares are 50% / 25% / 25%.
- Total enrolment limit
The total completion target divided by the completion rate, rounded up. It is the most participants that may be enrolled at once.
Worked example
Targets 50 / 25 / 25, rate 0.5:
randomisation shares 0.5 / 0.25 / 0.25
total completion target 100 participants
total enrolment limit 200 (100 ÷ 0.5)
How participants are assigned¶
Assignment is weighted random: a new participant is placed into an arm with probability equal to that arm’s share. An arm with twice the target gets twice the chance.
Two properties matter:
Targets are soft. An arm is never closed for being “full”; it can overshoot. So targets 50 / 25 / 25 might finish as, say, 67 / 23 / 38, that is expected and fine.
The shares are fixed. They do not rebalance as arms fill. The portal will not steer participants toward an under-filled arm to hit the targets exactly, so final counts scatter around the targets rather than landing on them.
If you genuinely want equal assignment across arms, give every arm the same target; equal targets produce equal shares.
When recruitment pauses or stops¶
Recruitment is governed by two trial-wide gates, never by a single arm.
- Enrolment limit, a reversible pause
When the number of participants currently enrolled (everyone who has not withdrawn) reaches the total enrolment limit, recruitment pauses. It resumes on its own as that number falls, for example when participants withdraw and free a place. A withdrawal refunds a place; a completion does not.
- Completion target, an irreversible stop
When the number of participants who have completed the trial reaches the total completion target, recruitment ends for good and does not reopen.
In a balanced trial you usually reach the completion target (the hard stop) before the enrolment limit (the soft pause). The pause mainly bites when completions are lagging , lots of people enrolled and in progress, but few finishing yet, which is exactly when you would want to stop pulling more people in.
Adjusting the plan while it runs¶
You can change the per-arm targets and the completion rate after launch, from the trial overview (see Running a trial). The derived total, limit, and shares recompute, and the gates use the new values from then on. This lets you retarget an under- or over-recruiting trial without rebuilding it.
Parallel vs crossover¶
The randomisation shares are defined per arm. In a parallel trial each participant is in one arm, so the shares apply directly. In a crossover trial every participant passes through all arms in a randomised order, so what is actually randomised is the order; the per-arm targets weight those orderings. If you run a crossover design, confirm the intended weighting with the trial statistician.