motief/thoughts/shared/designs/2026-04-16-glpvda-merger-svd-analysis-design.md

date	topic	status
2026-04-16	GroenLinks-PvdA Merger Dynamics in SVD Space	validated

Problem Statement

We need concrete, data-driven findings about the GroenLinks-PvdA merger for a blog post. Four questions:

1. How similar were GL and PvdA in SVD space before the merger?
2. How cohesive is the merged party compared to others?
3. When did GL and PvdA converge in SVD space?
4. Which parties shifted the most, and how do GL/PvdA compare?

Constraints

• Investigation-only: query the database, report findings, no code changes
• Database: data/motions.db (DuckDB), SVD vectors in svd_vectors table
• Party affiliation: use mp_votes.party for historical labels (not mp_metadata which only tracks current party)
• SVD vectors are at MP level (entity_type='mp'); party centroids must be computed from MP vectors
• Cross-window scale differences require normalization (distances as fraction of avg inter-party distance)

Approach

Method: Compute party centroids from MP-level SVD vectors, grouped by mp_votes.party for historical party labels. Normalize all distances as fractions of the average inter-party distance in each window to make cross-window comparisons meaningful.

Data source: svd_vectors table (MP vectors per window) joined with mp_votes (historical party labels per vote, majority party per MP per year).

Key discovery: mp_metadata only tracks current party — pre-merger GL and PvdA MPs who merged are now labeled "GroenLinks-PvdA". We must use mp_votes.party for historical accuracy.

Architecture

N/A — this is a data investigation, not a system design.

Components

Finding 1: GL-PvdA Pre-Merger Similarity

GL and PvdA were already remarkably close in SVD space well before the merger:

Year	GL↔PvdA Distance	As % of Avg Inter-Party Distance	Nearest Other Party	Distance to Nearest
2019	2.10	10.5%	PvdD	9.6
2020	2.23	5.0%	CU	28.7
2021	1.46	4.4%	FVD	11.6
2022	1.16	2.8%	FVD	6.5

The nearest non-PvdA party to GL was always 5-10x further away than PvdA itself. They converged over time — from 10.5% of average inter-party distance in 2019 down to 2.8% in 2022.

Finding 2: Post-Merger Cohesion

Year	GL-PvdA Spread	Avg Other Spread	Ratio	Cohesion Rank
2023	1.50	19.95	0.08	#1 most cohesive
2024	14.05	18.47	0.76	Mid-pack
2025	28.09	18.09	1.55	Below average
Current	43.30	28.05	1.54	Below average

The merged party started as the most cohesive party in parliament (2023), but by 2025 its internal spread is 55% above average — the merger created a party that's internally more diverse than typical Dutch parties.

Finding 3: Merger Convergence Timeline

Window	GL↔PvdA Distance	Normalized Ratio
2019-Q3	0.98	25.5%
2020-Q1	1.38	18.6%
2021-Q1	1.58	19.3%
2022-Q3	0.86	9.4%
2023-Q1	0.58	7.1%
2023-Q3	0.37	4.5%
2023-Q4	0.46	5.5%

By Q3 2023 — just before the formal merger — GL and PvdA centroids were only 4.5% of the average inter-party distance apart. Essentially indistinguishable in voting pattern space.

Finding 4: Large Positional Shifts

GL and PvdA were the most stable parties in parliament (normalized drift per year):

Period	GL Drift	PvdA Drift	VVD Drift	D66 Drift	PVV Drift
2019→2020	14.5%	16.6%	140.2%	145.6%	121.9%
2020→2021	21.8%	25.8%	115.8%	82.2%	207.3%
2021→2022	11.6%	10.8%	70.8%	91.2%	51.9%
2022→2023	54.5%	23.3%	109.7%	177.3%	222.1%

While VVD and D66 moved 70-177% per year, GL and PvdA drifted only 10-25%. The merger partners were anchored in place while the rest of the landscape shifted.

Data Flow

1. Query svd_vectors for MP vectors per window
2. Join with mp_votes to determine each MP's majority party in that year
3. Compute party centroids as mean of member vectors
4. Compute pairwise distances and normalize by average inter-party distance
5. Track convergence timeline using quarterly windows

Error Handling

• Windows with insufficient MPs (<3 per party) are excluded from centroid calculations
• The mp_votes.party column uses multiple label variants ("GroenLinks", "GL", "GroenLinks-PvdA") — normalized in queries
• The 2023 transition year has mixed labels (some GL, some PvdA, some GL-PvdA) — handled by majority-vote assignment per MP

Testing Strategy

N/A — data investigation. Key validation checks:

• Cross-reference MP counts with known parliament compositions
• Verify that GL + PvdA MP counts match expected seat counts per year
• Confirm that convergence timeline aligns with known political events (merger announcement Oct 2023)

Open Questions

• Should we compute cosine similarity instead of Euclidean distance for cross-window normalization?
• The 2025 and current_parliament windows show very different absolute scales — should we normalize vectors before computing distances?
• The few remaining "GL" (8) and "PvdA" (5) labeled MPs in 2025 may be artifacts — should they be included in the GL-PvdA group?