{
  "schema": "vedokrok.public-item.v1",
  "release_id": "MHC-RPUB-20260920-75ad787a",
  "url": "/knowledge/preserve-the-pair-in-a-before-and-after-comparison",
  "id": "MHC-D-RESEARCH-0271",
  "version": "0.1.0",
  "title": "Preserve the pair in a before-and-after comparison",
  "summary": "Two columns may contain a relationship that their separate averages conceal.",
  "kind": "protocol",
  "body": "Keep the correspondence between observations and examine the within-pair changes. A paired design is not the same as two unrelated samples. Verify identities and missing pairs before selecting an analysis appropriate to the design and data.",
  "limits": [
    "A before-and-after change alone does not isolate the intervention from time trends or other causes."
  ],
  "topics": [
    "union-statistical-judgment"
  ],
  "intents": [],
  "source_ids": [
    "RS-8D669AD19865FC1C"
  ],
  "evidence": [
    {
      "claim": "Paired observations have a meaningful one-to-one correspondence and can be analyzed through within-pair differences.",
      "source_id": "RS-8D669AD19865FC1C",
      "role": "supports",
      "note": "Pairing must reflect the study structure, not an arbitrary arrangement after seeing the results.",
      "locator": "Paired versus unpaired samples"
    }
  ],
  "use_when": [
    "The same units are measured twice or observations were deliberately matched."
  ],
  "avoid_when": [
    "A before-and-after change alone does not isolate the intervention from time trends or other causes."
  ],
  "example": "A person's later score should be compared with that person's earlier score, not an arbitrary row from a separately sorted list.",
  "check": "Every claimed change uses the correct pair, and missing pairs are not silently replaced.",
  "steps": [
    "Confirm what makes each pair meaningful.",
    "Link observations using reliable identifiers and inspect unmatched cases.",
    "Analyze changes with a method that respects the pairing and relevant assumptions."
  ],
  "sources": [
    {
      "id": "RS-8D669AD19865FC1C",
      "title": "Two-Sample t-Test for Equal Means",
      "url": "https://www.itl.nist.gov/div898/handbook/eda/section3/eda353.htm"
    }
  ],
  "relations": [
    {
      "from": "MHC-D-RESEARCH-0271",
      "to": "MHC-D-RESEARCH-0272",
      "type": "useful_with",
      "url": "/knowledge/do-not-mistake-frequent-readings-for-independent-evidence"
    }
  ],
  "collections": [
    {
      "id": "RC-D4FD0B28897E9517",
      "title": "Check what a study can actually tell you",
      "url": "/collections/check-what-a-study-can-actually-tell-you"
    }
  ]
}