---
title: Always Derive Blog Numbers from Pipeline Outputs, Not Memory
date: 2026-04-16
last_updated: 2026-04-24
category: docs/solutions/best-practices
module: documentation
problem_type: best_practice
component: documentation
severity: medium
applies_when:
  - Writing or updating a data-driven blog post
  - Adding EVR percentages, vote counts, correlation coefficients, or any quantitative claims
  - Referencing pipeline components (embeddings, fusion, similarity) in public-facing docs
tags: [blog, pipeline, evr, svd, canonical-outputs, data-driven-docs, reproducibility, correlation]
---

# Always Derive Blog Numbers from Pipeline Outputs, Not Memory

## Context

The political compass blog post was written with hardcoded numbers (EVR ~32%/~21%, 38 windows) that drifted from the pipeline's actual outputs as the data and methodology evolved. A maintenance session was required to bring every figure back in sync, generate supporting visuals, and strip references to pipeline components not yet deployed to production.

## Guidance

**Pull every quantitative claim directly from the canonical pipeline functions:**

| Claim | Canonical source |
|-------|-----------------|
| EVR percentages | `analysis.political_axis.compute_svd_spectrum(window_ids=[...])` |
| Vote/motion counts | `SELECT COUNT(*) FROM motions / mp_votes` via `data/motions.db` |
| Window count | `analysis.political_axis` — count of aligned windows |
| Party agreement | `analysis.explorer_data` or direct SQL on `mp_votes` |
| Correlation coefficients | Compute from canonical metrics in DB, never hardcode |

**Never reference pipeline components that are not in production.** If `fused_embeddings` rows exist in the DB but the fusion pipeline is not yet in active use, do not describe it as part of the current workflow in blog copy.

**Generate supporting visuals programmatically** (matplotlib → `docs/research/`) and embed them by relative path in the blog HTML. This makes regeneration trivial when numbers change.

## Why This Matters

Hardcoded numbers in blog copy inevitably drift from reality as:
- More parliamentary windows are added (38 → 41 → …)
- SVD methodology changes (e.g., Procrustes alignment, window selection)
- Pipeline components are added or removed from production

When numbers drift, the post loses credibility and requires an expensive archaeology pass to fix. Generating them from the pipeline makes each update a single script run.

## When to Apply

- Before publishing or updating any post that cites quantitative pipeline outputs
- When the pipeline has changed (new windows, new methodology) and existing posts reference old numbers
- When removing or adding a pipeline stage — audit all docs for references to that stage

## Examples

**Before (hardcoded, stale):**
```html
<p>PC1 explains ~32% of the variance and PC2 explains ~21% — together ~52%.</p>
```

**After (derived from pipeline, accurate):**
```python
# scripts/generate_blog_assets.py
from analysis.political_axis import compute_svd_spectrum

evr = compute_svd_spectrum(window_ids=["current_parliament"])
# evr[0] = 0.290, evr[1] = 0.1146 → PC1~29%, PC2~11.5%, total~41%
```
```html
<p>PC1 explains ~29% of the variance and PC2 explains ~11.5% — together ~41%.</p>
```

**Multi-window EVR (Procrustes-aligned across all 41 windows):**
```python
evr_multi = compute_svd_spectrum()  # no window_ids → all windows
# evr_multi[0] = 0.1463, evr_multi[1] = 0.1310
```

**Party agreement for a specific window:**
```python
import duckdb
con = duckdb.connect("data/motions.db")
# Agreement between two parties in a quarter
sql = """
  SELECT AVG(CASE WHEN a.vote = b.vote THEN 1.0 ELSE 0.0 END)
  FROM mp_votes a JOIN mp_votes b USING (motion_id)
  WHERE a.party = 'GroenLinks' AND b.party = 'PvdA'
    AND a.motion_id IN (SELECT id FROM motions WHERE window_id = '2023-Q3')
"""
```

**Correlation between voting extremity and policy extremity:**

- ❌ **Before (hardcoded, unverifiable):**
  ```html
  <p>De correlatie tussen beide maten is r = -0.011 — ze meten totaal verschillende dingen.</p>
  ```
  Problem: No script, query, or function reproduces this number. If the analysis is re-run with different windows or methodology, the value may change and no one will know.

- ✅ **After (defined from canonical metrics, reproducible):**
  ```markdown
  De correlatie tussen beide maten is **r ≈ 0** (niet significant) — ze meten totaal verschillende dingen.

  *Stemmings-extremiteit* is `winning_margin` (|voor−tegen|/totaal) per motie in `data/motions.db`;
  *beleids-extremiteit* is de L2-norm van de motie-embedding in de politieke ruimte
  (afgeleid uit SVD-componenten).
  ```
  The metrics are defined canonically. Anyone can recompute the correlation from the database.

## Related

- `docs/solutions/best-practices/svd-labels-voting-patterns-not-semantics.md` — companion guidance on keeping SVD axis *labels* aligned with voting data rather than semantic assumptions
