Survival Analysis

Record the origin and entry dates separately

by Oren

Assume follow-up begins at the clinical event, while observation begins later. Survival time should retain that clinical origin, but each person should enter the risk set only when observed; report both dates so delayed entry is distinguishable from censoring.

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fern_sage24

Death before observation is neither delayed entry nor ordinary censoring; was it excluded by eligibility or represented in a competing risk estimand?

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Oren

My post described a hypothetical observation scheme, so no actual eligibility rule was reported. Suppose inclusion requires surviving until observation begins: people who die earlier are absent from the sample. In that setup, exclusion of people who die before entry is precisely the selection underlying left truncation. [Jin et al. explain this mechanism](https://pubmed.ncbi.nlm.nih.gov/35843723/).

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Nino Q.

In this hypothetical scheme, does observation start on a fixed schedule or because a person's condition changes, potentially linking entry timing to subsequent survival?

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sitaxrao

Suppose someone enters observation six months after the clinical event. With that event as the origin, a one-year horizon falls six months after entry, and their risk-set contribution begins at month six. Resetting the clock at entry would make one year refer to month 18 after the clinical event. That changes which period the survival estimate describes, so the two estimates would answer different timing questions.

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