Anchors

An anchor is a per-participant reference date. Where a stage can be scheduled a fixed number of days after a participant joins, an anchor lets you schedule it relative to something that happens to that participant at their own time — the date they were randomised, their surgery date, their treatment start.

Every participant can hold a date for each anchor you define, and those dates are what stage schedules are calculated from.

Find them under Stages > Anchors.

When you need one

You need an anchor whenever "day 30" means thirty days after an event rather than thirty days after enrolment. Typical examples:

  • A surgical study where follow-up runs from the operation date, which differs for every participant.
  • A treatment study where the baseline is the first dose.
  • A study where participants are recruited over months but the schedule runs from a shared index event.

If everything in your study is scheduled from enrolment, you do not need anchors at all.

Creating an anchor

Click Add anchor and give it:

  • A name — required, and how you will pick it when scheduling stages. Name it after the event: "Surgery date", "Randomisation", "First dose".
  • A description — optional, but worth writing for whoever sets these dates in the clinic.
  • An assignment rule — how the date gets set for each participant.

Assignment rules

Rule Behaviour
Do not automatically trigger (manual only) Nothing sets it; a member of staff enters the date on the participant's record
When participant first logs in Set automatically the first time the participant opens the study
When a stage is completed Set automatically when a given stage is submitted — optionally a specific recurrence or anchor slot of it. The date is either the moment of submission, or a date written on the form (below)

By default, automatic rules set the anchor to the moment the trigger occurred. They only ever fire once per participant: if a participant already has a date for that anchor, the rule does not overwrite it. That means a manually-corrected date is safe from being reset by a later trigger.

Taking the date from a field on the form

The moment a form is submitted is often not the date that matters. Staff may record an operation days or weeks after it happened, so scheduling from the submission would put every follow-up in the wrong place.

For a When a stage is completed rule, choose Date to use, then pick one of the stage's date or date-and-time fields, such as "The date in "Procedure date"". The anchor is then set to the date written in that field, whenever the form was filled in. Only date and date-and-time fields on the chosen stage are offered.

Then choose When the date changes:

  • Follow the most recent date (the default). The anchor is the date most recently written in the field, on any of the participant's submissions of that form:
    • A new submission with a date moves the anchor.
    • Correcting the date on any submission, including an older one, moves it.
    • Clearing that date, or deleting its submission, moves the anchor back to the date written before it, or clears it if none is left.
    • Saving the form again without changing the date doesn't count as a new date.
  • Keep the first date. The first date entered sets the anchor, and it stays put. Later submissions, corrections and deletions don't move it. To change it, set it by hand on the participant's record.

Either way, saving the form with the date left blank does not set the anchor. It never falls back to the submission time. If the rule names a specific recurrence or anchor slot, only submissions of that one count.

Two things are never overwritten by the field:

  • A date set by hand. If you set or correct the anchor on the participant's record, the field stops controlling it from then on.
  • A date set before the rule named a field, for example by the submission-time version of the rule.

If the field is later taken off the stage, the Anchors tab flags the rule, because it can no longer set anything.

Setting a participant's anchor date

For manual anchors — and to correct an automatically-set one — open the participant's record and find the anchors section on their information page. Each anchor you have defined appears there with a date field. Set it, or clear it.

Setting a date immediately recalculates every stage scheduled against that anchor for that participant, so their schedule appears (or shifts) straight away.

Using anchors to schedule stages

Anchors do nothing on their own — they take effect through stages, in two places on a stage's Availability tab.

Anchor-scheduled stages. Choose Anchor-scheduled under Schedule stage, then add one slot per appearance: an anchor and a day offset. Offsets can be negative, so a pre-operative form can sit at day −7 while follow-ups sit at day 30 and day 90. Each slot appears to the participant as its own occurrence of the stage.

Make available after an anchor date. Under Stage availability, choose Make available after… and set the trigger to Anchor date, with an optional delay in days. The stage unlocks once the anchor is set and the delay has elapsed.

Until a participant's anchor date is set, stages that depend on it have nothing to schedule from and do not appear for that participant. This is normal — but it does mean a forgotten anchor date shows up as a participant with a suspiciously empty schedule.

Define anchors before you build the stages that use them. The stage editor can only offer anchors that already exist, and shows a warning where none are defined.

Editing and deleting

Editing an anchor's name, description or assignment rule is safe: dates already recorded against it are unaffected. Changing the assignment rule only affects participants who do not yet have a date. Switching a rule to take its date from a field does not reach back to forms already submitted: the anchor is set from the field the next time each participant's form is saved.

Deleting an anchor is not safe, and Trialflare warns you before it happens. It removes every scheduled stage that depends on that anchor, and clears the anchor date from every participant. There is no undo. If stages are already scheduled against an anchor on a live study, repoint those stages first and delete the anchor afterwards.

Permissions

To do this You need
View and manage anchors on the Anchors tab Trial administrator — trial.admin, or trials.manage at team level
Set or clear an individual participant's anchor date trial.writeParticipantStudyData (or trial-wide write access)
Reference anchors when configuring a stage trial.write

See Roles and permissions.