Disposal

[henrydingliu]

Summary of Changes

Related GitHub Issue(s)

Additional Context for Reviewers

• I passed tests locally for both code (uv run pytest) and documentation changes (uv run jb build docs --builder=custom --custom-builder=doctest)

---

Note

Medium Risk
Introduces a new reserving path that derives link ratios and full triangles from disposal rates and ultimates; incorrect use could materially change IBNR projections, though scope is limited to the new estimator plus a small shared base-class refactor.

Overview
Adds DisposalRate, a new adjustment estimator on DevelopmentBase that implements Friedland’s disposal-rate approach: observed cumulative counts (or similar) are divided by supplied ultimates (sample_weight), weighted/fitted like other development factors, and transform rebuilds ldf_ from fitted disposal patterns and future run-off so full_triangle_ can extend the lower triangle.

The class is exported from chainladder.adjustments and the top-level package, with a regression test on the Friedland GL insurer closed-claim sample (fitted disposal rates and incremental lower-triangle deltas).

Refactor: _param_property moves from IncrementalAdditive to a shared static helper on DevelopmentBase (used by disposal and incremental fitting); DevelopmentBase type hints switch from TriangleLike to Triangle.

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[cursor]

Cursor Bugbot has reviewed your changes and found 2 potential issues.

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[cursor]

Debug stub left in class

Low Severity

The new DisposalRate class ends with a _test method that ignores its arguments and always returns the string 'test'. Nothing in the repository calls it, so it looks like leftover scaffolding rather than part of the public API.

 

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[cursor]

Zero divisor in runoff scaling

Medium Severity

In transform, future runoff is computed as incr_disposal_ / ibnr_pct * ibnr_. Where cumulative disposal already reaches 100% (ibnr_pct is zero) or ibnr_ is zero, division yields NaNs or infinities that can propagate into ldf_ and full_triangle_ instead of zero runoff.

 

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[codecov]

Codecov Report

❌ Patch coverage is 85.89744% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.09%. Comparing base (0f59a1b) to head (40f977f).
⚠️ Report is 21 commits behind head on disposal_rate_method.

Files with missing lines	Patch %	Lines
chainladder/adjustments/disposal.py	81.35%	6 Missing and 5 partials ⚠️

Additional details and impacted files

@@                   Coverage Diff                    @@
##           disposal_rate_method    #1021      +/-   ##
========================================================
+ Coverage                 88.94%   89.09%   +0.14%     
========================================================
  Files                        89       91       +2     
  Lines                      5139     5228      +89     
  Branches                    660      666       +6     
========================================================
+ Hits                       4571     4658      +87     
- Misses                      398      400       +2     
  Partials                    170      170              

Flag	Coverage Δ
unittests	89.09% <85.89%> (+0.14%)	⬆️

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