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title	date	module	problem_type	component	severity	tags	applies_when
SVD Axis Stability and Overtone Shift are Independent Phenomena	2026-04-05	analysis	insight	motion-analysis	medium	[svd overtone-shift semantic-drift axis-stability parliamentary-analysis]	[Interpreting SVD axes over multiple time windows Comparing motion content across different parliamentary periods Understanding why stable axis labels don't guarantee stable motion content]

SVD Axis Stability and Overtone Shift are Independent Phenomena

Key Insight

When analyzing SVD axes across time windows, axis stability and overtone shift measure fundamentally different phenomena:

Phenomenon	What it Measures	How to Compute
Axis Stability	Whether the same motions/embeddings load on an axis	Lasso regression: `SVD_score ~ fused_embedding`, compare weight vectors via cosine similarity + Jaccard
Overtone Shift	How motion content evolves over time	Semantic gravity (weighted mean embedding) tracking via cosine distance

The implication: An axis can be "stable" (parties vote similarly across years) while its semantic content drifts dramatically (different motions define the axis).

Evidence

Analysis of 9 annual windows (2016-2026) revealed:

9 of 10 axes are stable (similarity > 0.7)
All stable axes show high overtone shift (1.3-1.97 cosine distance)
No inflection points detected — drift is gradual, not sudden

Example: Axis 1 (Coalition vs Opposition)

Metric	Value	Interpretation
Axis Stability	0.83	Coalition/opposition voting pattern is structurally consistent
Overtone Shift	1.47 avg, 1.97 max	Motion content defining "coalition" vs "opposition" has changed substantially

This means: PVV, VVD, NSC, BBB consistently vote together against SP, GL-PvdA, PvdD across all windows — but the specific motions that exemplify "coalition policy" in 2026 are semantically different from 2016.

Why This Matters

Axis labels are temporally bounded — "Rechts kabinetsbeleid versus links oppositiebeleid" accurately describes 2016-2026, but the underlying motions have evolved.
Cross-temporal comparison is valid structurally but not semantically — Party positions are comparable; motion content is not.
Axis 6 (Migration/Culture) is an exception — Low stability (0.35) suggests fundamental change in how this dimension is structured.

How to Analyze This

Use scripts/motion_drift.py:

uv run python scripts/motion_drift.py --db data/motions.db --output reports/drift

The script computes:

Axis stability: Lasso regression weights compared across windows
Overtone shift: Semantic gravity tracking
Inflection points: Sudden drift detection
Party trajectories: How parties move along stable axes

Prevention

When updating SVD themes:

Run scripts/motion_drift.py to check current overtone shift levels
Verify that theme descriptions match current motion content, not historical content
Monitor Axis 6 specifically for stability issues
Consider temporal weighting in visualizations — recent windows better represent current semantics

scripts/motion_drift.py — Analysis script
docs/research/2026-04-05-svd-overtone-shift-deep-dive.md — Deep analysis
reports/drift/report.md — Generated report

3.4 KiB Raw Blame History