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Add opt-in ndt_edge_width for unbounded ndt kernels - #1344

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@EItanm1999 EItanm1999 commented Sep 19, 2026 •

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  • The admissibility guard floors the log-likelihood below t - st, which is
    exact only for a compact uniform ndt kernel of half-width st (the
    convention every LAN *_st network follows). A kernel with unbounded
    support, e.g. Normal(t, st) where st is a standard deviation, carries
    real density below t - st, so flooring there discards likelihood the
    model genuinely assigns.
  • Adds ndt_edge_width to the likelihood config, moving the floor to
    t - ndt_edge_width * st. Declared per likelihood, so a blackbox exact
    likelihood and its LAN approximation for the same model can carry
    different edges, and a registered model ships the correct edge as its
    own default.
  • None means 1.0 and is byte-identical to the previous behaviour;
    verified bitwise against the parent commit for ddm, ddm_sdv, full_ddm
    and a flat-likelihood st model. Inert for models without st: the edge
    does not move for ddm or ddm_sdv.
  • Reachability is proven end-to-end both ways, since a config field that
    does not survive the merge path is invisible to users: via a
    user-supplied model_config (edge 0.4 -> 0.2 at t=0.5, st=0.1, freeing
    10 trials from the lower bound) and via a registered model's own
    likelihood entry, which produces a bitwise-identical result. A
    user-supplied value overrides the registry default.
  • Because Config.from_defaults already splats **loglik_config, a
    likelihood-level key needs no plumbing there and register_model
    forwards it inside likelihoods verbatim, so no registration changes
    are required.
  • full_ddm reaches this guard but its visible edge is its own
    likelihood's: hddm_wfpt returns zero density below the full-width edge
    t - st/2, which sits above t - st, so widening the floor frees nothing
    and full_ddm is bitwise unchanged at any setting.

Stacked on #1292 (first commit of this branch is #1292's). Review only the top commit; this PR shrinks to one commit once #1292 merges. The ddm_normal_st registration (separate PR) depends on this field.

Tests: the branch's own test files pass on current main (fefed57) in an environment with ssm-simulators 0.14.0 and bambi 0.21; see the commit for the added cases. Fork CI has not been approved for this fork, so no check-runs appear here.

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Summary by CodeRabbit

  • New Features

    • Added the optional ndt_edge_width setting to control response-time admissibility for models with non-decision-time variability.
    • The default floor is one st below t; wider boundaries, such as three st, can be configured for unbounded distributions.
    • Models without st remain unaffected.
    • Invalid values, including negative, non-finite, or non-numeric widths, are rejected.
  • Documentation

    • Updated likelihood configuration guidance and the changelog with the new setting, defaults, and supported use cases.

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📝 Walkthrough

Walkthrough

The change adds configurable response-time admissibility floors for models with trial-to-trial non-decision-time variability. The floor uses t - ndt_edge_width * st, propagates through model construction and likelihood evaluation, and is covered by configuration and distribution tests.

Changes

Non-decision-time admissibility

Layer / File(s) Summary
Configuration contract
src/hssm/_types.py, src/hssm/config.py, tests/test_config.py
Adds optional ndt_edge_width fields, validates finite non-negative values, and verifies configuration propagation.
Distribution support guard
src/hssm/distribution_utils/dist.py, tests/distribution_utils/test_distribution_utils.py
Computes the floor from st and edge_width, applies it to lapse and non-lapse paths, and tests multiple parameter combinations.
Model integration and documentation
src/hssm/hssm.py, docs/tutorials/likelihoods.ipynb, docs/changelog.md
Passes the configured width into distribution creation and documents the floor behavior, defaults, and kernel-specific guidance.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant HSSM
  participant make_distribution
  participant HSSMDistribution
  participant ensure_positive_ndt
  HSSM->>make_distribution: pass ndt_edge_width
  make_distribution->>HSSMDistribution: resolve edge width
  HSSMDistribution->>ensure_positive_ndt: pass likelihood data and edge width
  ensure_positive_ndt-->>HSSMDistribution: apply admissibility floor
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Suggested reviewers: alexanderfengler

Merge Risk: 🟡 Moderate · up to 3b620

Invalid direct configuration values can produce incorrect likelihood support behavior, while the changelog contains unresolved documentation problems. These should be addressed before merging.

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Check name Status Explanation
Description Check ✅ Passed Check skipped - CodeRabbit’s high-level summary is enabled.
Title check ✅ Passed The title clearly summarizes the main change: adding opt-in ndt_edge_width support for unbounded non-decision-time kernels.
Docstring Coverage ✅ Passed Docstring coverage is 94.74% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 19 functions across 6 files.
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Actionable comments posted: 1


  • 🪄 Fix CodeRabbit comments on this PR
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In `@src/hssm/config.py`:
- Around line 246-247: Validate ndt_edge_width as finite and non-negative before
assigning it in ModelConfig, while preserving zero as valid. Apply the same
validation in Config.validate and at the make_distribution boundary so direct
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Comment thread src/hssm/config.py Resolved
@EItanm1999
EItanm1999 force-pushed the feat/ndt-edge-width-config branch from 2862fe0 to 4c2f2ff Compare September 20, 2026 00:10

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In `@docs/changelog.md`:
- Line 7: Replace the `#XXXX` placeholder in the changelog entry with the final
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Comment thread docs/changelog.md Resolved
@EItanm1999
EItanm1999 force-pushed the feat/ndt-edge-width-config branch from 4c2f2ff to b81d0c1 Compare September 20, 2026 02:59

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🟡 Minor · Pass the configured edge width to make_distribution. · dist.py:754-761

src/hssm/distribution_utils/dist.py:754-761
🎯 Functional Correctness | 🟡 Minor | ⚡ Quick win

Pass the configured edge width to make_distribution. When a registered likelihood sets ndt_edge_width to a value other than 1.0, this helper omits it and make_distribution uses its 1.0 fallback. For ndt_edge_width=3.0, responses between the configured t - 3*st edge and fallback t - st edge receive LOGP_LB instead of the likelihood output.

        ndt_edge_width=config.ndt_edge_width,
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In `@src/hssm/distribution_utils/dist.py` around lines 754 - 761, Update the
make_distribution call in the distribution-construction helper to pass
config.ndt_edge_width, preserving the configured edge width for registered
likelihoods instead of relying on make_distribution’s default.

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Inline comments:
In `@docs/changelog.md`:
- Line 7: Update the changelog entry’s description of NaN and infinite
ndt_edge_width values to state that _validate_ndt_edge_width rejects non-finite
widths with ValueError, removing the claim that they change likelihood support
without an error.

---

Outside diff comments:
In `@src/hssm/distribution_utils/dist.py`:
- Around line 754-761: Update the make_distribution call in the
distribution-construction helper to pass config.ndt_edge_width, preserving the
configured edge width for registered likelihoods instead of relying on
make_distribution’s default.

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Comment thread docs/changelog.md Resolved
ensure_positive_ndt floors the log-likelihood wherever rt - t <= 1e-15. That is
the correct support edge for a fixed non-decision time, but not for the LAN
`*_st` models, which follow the half-width st convention of ssms: there t is
drawn per trial from Uniform(t - st, t + st), so the fastest admissible response
time is t - st and the band [t - st, t] carries real density. The guard replaced
that entire band with LOGP_LB regardless of what the likelihood returned.
Applied unconditionally at both call sites, affecting every LAN `*_st` model.

Measured on ddm_st: compiling HSSM's observed-RV logp for a model whose
likelihood IS the exact ddm_st quadrature and comparing against the same
quadrature called directly gave differences of -5.3 to -12586.8 nats, varying
with theta. With the guard skipped the two agree to exactly 0.0000 on 7 of 8
parameter vectors (the 8th had a subject z outside its bound). So this guard
accounts for the entire discrepancy while parameters stay in bounds.

p_outlier partially masks it: the floored value is wrapped in the lapse mixture,
so affected trials emerge at log(0.05/20) = -5.99 rather than at LOGP_LB, which
is why this presented as bad geometry rather than an obvious -inf.

The t - st edge is stated as correct under the half-width convention of
ssms/cssm, which every LAN *_st model follows, rather than as a universal law.
full_ddm is the one bundled model on the other convention: its only likelihood
is the blackbox wrapper around hddm_wfpt, which reads st as a full width and
puts its own edge at t - st/2. No separate factor is needed, because that edge
sits above t - st and hddm_wfpt already returns zero density across
[t - st, t - st/2), which the wrapper maps to the same lower bound. Verified:
across a 0.28-0.46 RT scan the raw likelihood's highest floored rt is exactly
t - st/2, and the guard changes no value at any rt (max |delta| 0.0).

Only st moves the response-time support edge. sz (starting point) and sv (drift)
do not, and are deliberately not consulted; a regression test pins that down.

Tests are one parametrized case per convention (fixed t, st moves the edge,
sz/sv leave it) over an explicit RT vector that straddles all three edges, so
the in-band assertion no longer depends on an unseeded draw. They compare
against the imported LOGP_LB rather than a -66.1 literal: under
PYTENSOR_FLAGS=floatX=float32 the bound is -66.0999984741211, and wrapping it in
np.array defeats NumPy's weak promotion, so the literal form failed there.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
@EItanm1999
EItanm1999 force-pushed the feat/ndt-edge-width-config branch from 7cc0cfd to 3b620f7 Compare September 23, 2026 00:23

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🟠 Major · Validate ndt_edge_width in make_distribution. · dist.py:569

src/hssm/distribution_utils/dist.py:569
🎯 Functional Correctness | 🟠 Major | ⚡ Quick win

Validate ndt_edge_width in make_distribution.

A direct caller can bypass Config.validate. NaN and positive infinity disable the support-floor comparison for finite st; negative infinity floors every response time. A negative finite value moves the floor above t. Bool values also bypass the declared validation policy. Apply the same finite, non-negative, non-bool validation before assigning _ndt_edge_width.

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In `@src/hssm/distribution_utils/dist.py` at line 569, Update make_distribution to
validate ndt_edge_width before assigning _ndt_edge_width: accept None or a
non-bool, finite, non-negative numeric value, and reject NaN, infinities,
negative values, and bools using the existing validation behavior where
applicable.

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Outside diff comments:
In `@src/hssm/distribution_utils/dist.py`:
- Line 569: Update make_distribution to validate ndt_edge_width before assigning
_ndt_edge_width: accept None or a non-bool, finite, non-negative numeric value,
and reject NaN, infinities, negative values, and bools using the existing
validation behavior where applicable.

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- The admissibility guard floors the log-likelihood below t - st, which is
  exact only for a compact uniform ndt kernel of half-width st (the
  convention every LAN *_st network follows). A kernel with unbounded
  support, e.g. Normal(t, st) where st is a standard deviation, carries
  real density below t - st, so flooring there discards likelihood the
  model genuinely assigns.
- Adds ndt_edge_width to the likelihood config, moving the floor to
  t - ndt_edge_width * st. Declared per likelihood, so a blackbox exact
  likelihood and its LAN approximation for the same model can carry
  different edges, and a registered model ships the correct edge as its
  own default.
- None means 1.0 and is byte-identical to the previous behaviour;
  verified bitwise against the parent commit for ddm, ddm_sdv, full_ddm
  and a flat-likelihood st model. Inert for models without st: the edge
  does not move for ddm or ddm_sdv.
- Reachability is proven end-to-end both ways, since a config field that
  does not survive the merge path is invisible to users: via a
  user-supplied model_config (edge 0.4 -> 0.2 at t=0.5, st=0.1, freeing
  10 trials from the lower bound) and via a registered model's own
  likelihood entry, which produces a bitwise-identical result. A
  user-supplied value overrides the registry default.
- Because Config.from_defaults already splats **loglik_config, a
  likelihood-level key needs no plumbing there and register_model
  forwards it inside likelihoods verbatim, so no registration changes
  are required.
- full_ddm reaches this guard but its visible edge is its own
  likelihood's: hddm_wfpt returns zero density below the full-width edge
  t - st/2, which sits above t - st, so widening the floor frees nothing
  and full_ddm is bitwise unchanged at any setting.

Validation lives in `distribution_utils.dist` and runs at both boundaries:
`Config.validate()` and a direct `make_distribution()` call, which previously
accepted negative and non-finite widths and passed them to the guard.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
@AlexanderFengler

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Thanks, Eitan. The knob was the right instinct, and I want to explain why it went into #1292 in a different shape rather than this PR being merged. I'm closing this one as superseded; nothing in it is lost.

The floor logic here is correct, and the default path is bit-for-bit identical to #1292 (I checked 8 model × lapse combinations, plus ddm/ddm_sdv, where it has no effect). What I didn't want to ship is the rule underneath both PRs: ensure_positive_ndt changed behaviour based on whether a parameter named st exists, and ndt_edge_width then scaled that name-keyed edge. st means three different things across our models (half-width for the ssms LANs, full width for full_ddm's hddm_wfpt likelihood, a standard deviation for ddm_normal_st), so a rule keyed on the name is silently wrong for some of them, and a model whose variability parameter has another name gets no edge at all.

So #1292, now merged (32b7e23), carries a declared edge instead: a likelihood config says which parameter shifts the support edge and by how much, "ndt_edge_shift": {"param": "st", "scale": 0.5} (full_ddm), and the guard applies the resolved edge t − scale·param without knowing any parameter names. Absent the declaration, the floor sits at t, exactly as before. The validation you added (param must be in list_params, t must exist, finite non-negative scale) and the update_config override path (hssm.HSSM(..., model_config={"ndt_edge_shift": ...}) beats the registry entry) are carried over as hssm._types.NDTEdgeShift, and make_distribution_for_supported_model forwards the declaration, which closes the forwarding gap CodeRabbit flagged here and on #1345. The ### Unreleased changelog entry on main credits this PR for the configurable edge, and the mechanism is documented in docs/explanations/likelihoods.md ("Declaring where the response-time support starts").

For your two model PRs:

Open question

  • For an unbounded kernel like ddm_normal_st (ssm-simulators 0.15.0 declares no edge for it, so the 3σ floor is an HSSM-side policy), did you compare no floor at all (or only rt <= 0) with the 3σ edge? The validator rejects inf, so "no floor" still can't be expressed; if there is a case for it we can allow it in a follow-up.

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