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State that ML-UMR is for single-arm, fully disconnected evidence - #118

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@choxos choxos commented Sep 29, 2026 •

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Documentation only; no code changes. The README and vignettes now state throughout that ML-UMR is for single-arm indirect comparisons of fully disconnected evidence, and two results that could be misread are explained.

README

  • Opening paragraph. ML-UMR is described as a population-adjusted single-arm indirect treatment comparison for treatments from fully disconnected evidence, extending ML-NMR to the single-arm, fully unanchored setting.
  • Estimator list. STC is an unanchored simulated treatment comparison (was "one-arm"). The naive benchmark is an "unadjusted comparison of outcomes across studies".
  • "When to use ML-UMR" table. MAIC and STC are "Anchored or fully unanchored (single-arm)". ML-UMR is "Fully unanchored (single-arm)".
  • "Most appropriate when" list. Item 2 now reads "Indirectly comparing treatments from single-arm trials (i.e., fully disconnected evidence where no common reference arm connects the evidence)".
  • New scope paragraph after that list:
    • ML-UMR is only for fully unanchored, single-arm comparisons.
    • Randomized trials call for ML-NMR or another appropriate method.
    • The vignettes' examples build hypothetical single-arm trials only for illustration.
  • Marginal hazard ratio paragraph. It now states that the ratio varies over time even when both studies share one baseline shape (aux_by = "none"):
    • Why: a shared shape makes the conditional ratio constant, but the two risk sets lose high-risk patients at different rates. The hazard ratio is not collapsible.
    • A worked number: with a common Weibull shape of 1.5, the marginal ratio rises from 0.50 to 0.66 while the conditional ratio stays at 0.50.
    • When it stays constant: no prognostic covariate, or no treatment difference.
  • Wording elsewhere. The package-choice line, the namespace example and the References paragraph use the same single-arm, fully disconnected wording.

Vignettes

  • Examples built on randomized-trial data (binary-outcomes, continuous-outcomes, count-outcomes, survival-outcomes, fitting-and-diagnostics, choosing-a-method). Each data note now says three things:
    • The example creates hypothetical single-arm trials only to illustrate ML-UMR. It does this by dropping the common reference arm (placebo, or placebo and etanercept) or by treating one trial's arms as separate sources.
    • Randomized trials should never be analyzed this way in practice; ML-NMR or another appropriate method should be used.
    • ML-UMR is only for fully unanchored, single-arm comparisons.
  • introduction. States the same scope once for all the examples.
  • choosing-a-method. Retitled "Choosing a method for single-arm indirect comparisons" (vignette index entry and pkgdown navbar too). It now says in the opening, the methods table, the dataset section, the comparison table and the decision guide that every method covered is for single-arm indirect comparisons of fully disconnected evidence.
  • count-outcomes. Explains why the SPFA rate ratio is identical in the index population, the comparator population and at every covariate profile:
    • Under SPFA with a Poisson log link, the rate ratio is directly transportable in the sense of the cited transportability paper (arXiv:2602.17041).
    • The shared covariate factor cancels, and mlumr standardizes rates per unit exposure.
    • The target population still matters for absolute predictions, for the relaxed model, and for non-collapsible measures such as the odds and hazard ratios.
  • Transportability reference. Now cites the arXiv preprint (doi:10.48550/arXiv.2602.17041) instead of an unpublished manuscript. This changes the reference list of every vignette that cites it.
  • NEWS.md. New bullet under "Example data and documentation".

Checks

  • Re-rendered HTML. The precompiled vignette HTML was re-rendered from the knitted .Rmd files, with no model refits.
    • The word-level HTML diff contains only the edited prose, the updated citation, and the duplicate library() lines. Those lines were already removed from the vignette sources but had not been rendered into the HTML.
    • Every .Rmd.orig and its knitted .Rmd carry the same edits.
  • Package build. R CMD build from a clean clone creates all nine vignettes and registers the new choosing-a-method title.
  • Hazard-ratio example. Computed numerically: one standard normal covariate with log hazard ratio 0.8, common Weibull shape 1.5, conditional HR 0.497. The marginal HR at t = 0.05, 0.5, 1, 2, 3 is 0.498, 0.530, 0.566, 0.619, 0.655. It stays at 0.497 with no prognostic effect.
  • Rate-ratio explanation. Checked against the Poisson SPFA generated quantities, where the covariate term cancels in both delta_index and delta_comparator. The shipped tables show all four rate ratios (both populations and three profiles) at 0.8118294.

Summary by CodeRabbit

  • Documentation
    • Clarified that ML-UMR applies to fully disconnected, unanchored single-arm comparisons. For randomized or connected evidence, use the randomized comparison, ML-NMR, or another suitable method.
    • Updated examples to explain that separating arms or omitting shared reference arms from randomized trials creates hypothetical single-arm evidence for illustration only.
    • Expanded guidance on STC target populations, count-outcome rate ratios across populations and covariate profiles, and time-varying marginal hazard ratios.
    • Updated method-selection guidance, article labels, and reference details.

README
- The opening describes ML-UMR as a population-adjusted single-arm indirect
  treatment comparison for treatments from fully disconnected evidence, and
  the extension of ML-NMR as one to the single-arm, fully unanchored setting.
- STC is described as an unanchored (not one-arm) simulated treatment
  comparison, and the naive benchmark as an unadjusted comparison of outcomes
  across studies.
- The "When to use ML-UMR" table gives MAIC and STC as anchored or fully
  unanchored (single-arm) and ML-UMR as fully unanchored (single-arm); the
  second "most appropriate when" item now reads "Indirectly comparing
  treatments from single-arm trials". A new paragraph states that ML-UMR is
  only for fully unanchored, single-arm comparisons and that randomized trials
  call for ML-NMR or another appropriate method.
- The marginal hazard ratio paragraph states that the ratio varies over time
  even when both studies share one baseline shape (aux_by = "none"), explains
  why (non-collapsibility: the two risk sets lose high-risk patients at
  different rates), gives a worked number (conditional HR 0.50, marginal HR
  0.50 rising to 0.66 with a common Weibull shape of 1.5), and names the cases
  where it stays constant.

Vignettes
- Every vignette built on randomized-trial data (binary, continuous, count,
  survival, fitting-and-diagnostics, choosing-a-method) says that the example
  creates hypothetical single-arm trials by dropping a common reference arm, or
  by treating a trial's arms as separate sources, only to illustrate ML-UMR;
  that randomized trials should never be analyzed this way in practice; and
  that ML-UMR is only for fully unanchored, single-arm comparisons. The
  introduction states the same scope once for all of them.
- choosing-a-method is retitled "Choosing a method for single-arm indirect
  comparisons" and states throughout that its methods are for single-arm
  indirect comparisons of fully disconnected evidence.
- count-outcomes explains why the SPFA rate ratio is the same in both
  populations and at every covariate profile: under SPFA with a Poisson log
  link it is directly transportable, the shared covariate factor cancels, and
  rates are standardized per unit exposure. It also says where the target
  population still matters (absolute predictions, the relaxed model, and
  non-collapsible measures such as the odds and hazard ratios).
- The transportability reference now cites its arXiv preprint
  (arXiv:2602.17041) instead of an unpublished manuscript.
- The precompiled HTML is re-rendered from the knitted sources without
  refitting; this also brings in the earlier removal of duplicate library()
  calls, which had not been rendered into the HTML.
Copilot AI balanced review requested due to automatic review settings September 29, 2026 18:23

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📝 Walkthrough

Walkthrough

The documentation scopes ML-UMR to fully unanchored, disconnected single-arm comparisons and identifies randomized-trial examples as hypothetical. It also explains SPFA count-rate-ratio invariance and time-varying marginal hazard ratios.

Changes

Single-arm comparison scope

Layer / File(s) Summary
Define the comparison setting
README.md, vignettes/introduction.Rmd, vignettes/choosing-a-method.*, _pkgdown.yml
The documentation specifies that ML-UMR and the methods in the selection vignette concern fully unanchored, disconnected single-arm evidence. It directs connected randomized evidence to ML-NMR or another appropriate method. The method-selection title and navigation label now identify the single-arm scope.

Example and model explanations

Layer / File(s) Summary
Qualify examples and vignette materials
vignettes/*-outcomes.Rmd*, vignettes/*-outcomes.html, vignettes/fitting-and-diagnostics.*, vignettes/subgroup-identification.html, inst/REFERENCES.bib
The vignettes describe randomized trial arms presented as disconnected single-arm evidence as hypothetical examples. Rendered examples remove package-loading calls in several setup sections. Reference entries add preprint details and DOI links. The subgroup simulation text reports the stated sampling and interval results for ovat_age_6.
Explain population-level effect results
NEWS.md, README.md, vignettes/count-outcomes.*
The count-outcomes explanation states that, under SPFA with a Poisson log link, the rate ratio is the same across target populations and covariate profiles. It distinguishes this result from absolute predictions and other effect measures. The survival discussion states that marginal hazard ratios can vary over time even with a shared baseline shape.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~15 minutes

Change: Other

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Title check ✅ Passed The title clearly and concisely states the primary documentation change: ML-UMR applies to single-arm, fully disconnected evidence.
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Comment thread README.md Outdated
The README and the choosing-a-method decision guide said randomized trials
should never be analyzed as single-arm evidence. Randomized trials can sit in
disconnected components (A vs C and B vs D with no link between C and D), where
randomization supplies no anchor for A vs B and ML-NMR cannot estimate it; that
evidence is fully disconnected, which is the setting ML-UMR is for. Both
sentences, and the NEWS bullet, now say that trials connected through a common
arm must not be broken into single-arm evidence. The notes in the worked
examples already refer to that specific step (dropping a common reference arm
or splitting one trial's arms) and are unchanged.
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choxos commented Sep 29, 2026

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Codex Review: Didn't find any major issues. Delightful!

Reviewed commit: a4d112dab3

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⚠️ Outside diff range comments (1)

🟡 Minor · Qualify at_time for shared baseline shapes. · README.md:150-156

README.md:150-156
🎯 Functional Correctness | 🟡 Minor | ⚡ Quick win

Qualify at_time for shared baseline shapes.

The README does not state that at_time applies only when the studies have different baseline shapes. For a shared baseline shape, marginal_effects(effect = "hr", at_time = <nonzero>) raises an error. Users must use predict(type = "loghr") for the time-varying curve.

Suggested fix
-The scalar marginal hazard ratio is therefore its value at one time, chosen with `at_time`, and the primary
+When the studies have different baseline shapes, the scalar marginal hazard ratio is its value at one time, chosen with `at_time`. With a shared baseline shape, it is the closed-form `t -> 0` limit (`at_time = 0`). The primary
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Review comment at @README.md around lines 150 - 156:
Update the README description near the scalar marginal hazard ratio to clarify
that `at_time` selects the evaluation time only when studies have different
baseline shapes; for a shared baseline shape, state that the result is the
closed-form limit at `at_time = 0` and direct users to `predict(type = "loghr")`
for the time-varying curve.

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Outside diff comments:
Review comments at @README.md:
- Around line 150-156: Update the README description near the scalar marginal
hazard ratio to clarify that `at_time` selects the evaluation time only when
studies have different baseline shapes; for a shared baseline shape, state that
the result is the closed-form limit at `at_time = 0` and direct users to
`predict(type = "loghr")` for the time-varying curve.

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The README said the scalar marginal hazard ratio is its value at a time chosen
with `at_time`. That holds only when the two studies have different baseline
shapes. With a shared shape the scalar is the closed-form t -> 0 limit,
marginal_effects() accepts only `at_time = 0`, and for an SPFA fit the value
equals the conditional hazard ratio. The sentence now says both, and points to
predict(type = "loghr") for the curve.
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choxos commented Sep 29, 2026

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@coderabbitai Thanks, valid: with a shared baseline shape the scalar HR is the closed-form t -> 0 limit and marginal_effects() rejects a nonzero at_time (R/predict.R, the shared-shape branch). Fixed in the latest commit: the README now says at_time selects the evaluation time only under study-specific shapes, that a shared shape gives the t -> 0 limit (only at_time = 0 accepted; equal to the conditional HR for an SPFA fit), and points to predict(type = "loghr") for the curve. @coderabbitai review

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Codex Review: Didn't find any major issues. Bravo.

Reviewed commit: 6d297a8805

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@choxos, thanks for the clarification and the fix. I’ll review the updated PR.


✏️ Learnings added
Learnt from: choxos
URL: https://github.com/choxos/mlumr/pull/118

Timestamp: 2026-09-29T18:41:15.070Z
Learning: In `R/predict.R`, when survival studies share a baseline shape, `marginal_effects()` returns the closed-form time-zero limit for the scalar marginal hazard ratio and rejects nonzero `at_time`. For an SPFA fit, this limit equals the conditional hazard ratio. Use `predict(type = "loghr")` for the time-varying curve.

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@choxos
choxos merged commit 95c5bbd into main Sep 29, 2026
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choxos deleted the docs-single-arm-scope branch September 29, 2026 20:02
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