[Stack 17/17] Fix K-means k divergence: preserve vote-encounter row order#2524
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jucor wants to merge 1 commit intospr/edge/c3450b9afrom
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[Stack 17/17] Fix K-means k divergence: preserve vote-encounter row order#2524jucor wants to merge 1 commit intospr/edge/c3450b9afrom
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This was referenced Mar 30, 2026
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## Summary - Fix K-means k divergence between Python and Clojure by preserving vote-encounter order for participant rows in the rating matrix - Python was using `natsorted()` (PID-numeric order) while Clojure's NamedMatrix preserves insertion order — different row ordering cascades into different first-k-distinct initialization seeds for group-level k-means - On vw: Python picked k=4 (wrong), Clojure picks k=2 — now both pick k=2 with identical cluster memberships ## Investigation findings The divergence chain: rating_mat row order → PCA projection order → base-cluster ID assignment → group k-means first-k-distinct init → different local optima → different silhouette landscape → different k. PCA components are identical (cosine similarity = 1.0), silhouette implementation matches, k-means algorithm matches — only the data ORDER feeding first-k-distinct differed. ## Changes - `conversation.py`: `update_votes()` preserves vote-encounter order for participant rows instead of `natsorted()` - `conversation.py`: `_apply_moderation()` preserves row order with list comprehension - Column (comment ID) ordering remains `natsorted` — doesn't affect clustering - Re-recorded vw cold-start blob and golden snapshots - Updated ordering tests, removed `test_group_clustering` xfail - Added `scripts/investigate_k_divergence.py` diagnostic tool ## Cold-start blob results | Dataset | Clj k | Py k | Match | |---------|-------|------|-------| | vw | 2 | 2 | exact (sizes [50,17]) | | biodiversity | 2 | 2 | exact (sizes [81,19]) | | bg2018 | 2 | 2 | close ([51,49] vs [52,48]) | | FLI | 2 | 3 | inherent PCA divergence (94.5% NaN, sil gap 0.001) | ## Test plan - [x] All 297 tests pass (0 failures, 58 xfailed) - [x] vw cold-start: k=2 exact match with Clojure blob - [x] biodiversity cold-start: k=2 exact match - [x] Ordering tests updated to expect encounter order - [ ] Re-record private dataset golden snapshots after stack rebase 🤖 Generated with [Claude Code](https://claude.com/claude-code) ## Squashed commits - Fix K-means k divergence: preserve vote-encounter order for participant rows - Update plan and journal: K-divergence investigation resolved - Remove investigation script (one-off diagnostic, not production code) - Rename k-divergence doc: investigation record, not a handoff - Update references to renamed investigation doc commit-id:4598a0a1
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Summary
natsorted()(PID-numeric order) while Clojure's NamedMatrix preserves insertion order — different row ordering cascades into different first-k-distinct initialization seeds for group-level k-meansInvestigation findings
The divergence chain: rating_mat row order → PCA projection order → base-cluster ID assignment → group k-means first-k-distinct init → different local optima → different silhouette landscape → different k.
PCA components are identical (cosine similarity = 1.0), silhouette implementation matches, k-means algorithm matches — only the data ORDER feeding first-k-distinct differed.
Changes
conversation.py:update_votes()preserves vote-encounter order for participant rows instead ofnatsorted()conversation.py:_apply_moderation()preserves row order with list comprehensionnatsorted— doesn't affect clusteringtest_group_clusteringxfailscripts/investigate_k_divergence.pydiagnostic toolCold-start blob results
Test plan
🤖 Generated with Claude Code
Squashed commits
commit-id:4598a0a1
Stack: