EvidenceVariable — PICO component
The formal definition of a clinical research variable: population, exposure, comparator or outcome. Referenced by Evidence to structure a PICO question.
Purpose of the resource
EvidenceVariable is the atomic unit of a research question. An
EvidenceVariable can represent a population (adults ≥ 18 years with non-valvular atrial
fibrillation), an intervention (apixaban 5 mg twice daily), a comparator (warfarin), or an
outcome (major vascular event at 12 months). Each variable carries detailed
characteristic — inclusion/exclusion criteria as DataRequirement or
TriggerDefinition — and its handling (continuous, dichotomous, ordinal,
time-to-event).
Key fields
| Field | Type | Cardinality | Role |
|---|---|---|---|
url | uri | 0..1 | Canonical URL. |
identifier | Identifier[] | 0..* | Business identifiers. |
version | string | 0..1 | Version. |
name | string | 0..1 | Technical name. |
title | string | 0..1 | Title. |
shortTitle | string | 0..1 | Short title. |
status | code | 1..1 | draft | active | retired | unknown. |
actual | boolean | 0..1 | Characterises an observed (true) or hypothetical (false) population. |
description | markdown | 0..1 | Description. |
characteristic | BackboneElement[] | 0..* | Criteria defining the variable. |
handling | code | 0..1 | continuous | dichotomous | ordinal | polychotomous. |
category | BackboneElement[] | 0..* | Variable categories (for nominal variables). |
JSON example
{
"resourceType": "EvidenceVariable",
"id": "stroke-event",
"url": "http://example.org/EvidenceVariable/stroke-event",
"status": "active",
"name": "StrokeEvent",
"title": "Stroke event within 12 months",
"description": "Any stroke event (ischemic or hemorrhagic) within 12 months following randomisation.",
"handling": "dichotomous",
"characteristic": [{
"definitionCodeableConcept": {
"coding": [{
"system": "http://snomed.info/sct",
"code": "230690007",
"display": "Cerebrovascular accident"
}]
}
}]
}Common pitfalls
- Non-discriminating characteristic — always point to DataRequirement or TriggerDefinition.
- Missing handling — without this information, the variable cannot be consumed by a statistical tool.
- Population vs outcome confusion — the same EvidenceVariable cannot fill both roles in a single Evidence.
Related resources
- Evidence — uses the variable.
- EvidenceReport.
- ResearchStudy.
- Group — concrete cohort.
- FHIR R5 hub.