---
date: 2026-03-29
topic: "Bootstrap confidence intervals and data enrichment"
status: validated
---

# Bootstrap Confidence Intervals & Data Enrichment

## Problem Statement

The SVD axis charts show party centroid scores as point estimates with no indication of reliability. Volt (N=1) and D66 (N=49) look equally confident. Additionally:
- 2016–2018 motions lack body text, weakening embedding quality for those windows
- `party_svd_scores.json` is a stale ad-hoc file missing NSC — should be deleted

## Constraints

- No re-SVD per bootstrap replicate — too expensive, only centroid uncertainty needed
- Single-window bootstrap only — party scores come from `current_parliament` raw SVD vectors, not the Procrustes pipeline
- Functional Python, using existing patterns (uv, duckdb, numpy)
- Don't break existing Streamlit rendering — error bars are additive
- Fixed random seed for reproducibility

## Approach

**Single-window centroid bootstrap.** For each party, resample its N MPs with replacement 1000×, recompute centroid per replicate, take percentile CIs. Cheap (no re-SVD needed), directly answers "how reliable is this score?".

Rejected alternatives:
- Multi-window Procrustes bootstrap: 1000× SVD cost, requires orientation canonicalization. Overkill.
- Analytical SE (std/sqrt(N)): assumes normality, misses skewed distributions.

## Components

### A. Download Script Enhancement (`scripts/download_past_year.py`)

Add two CLI flags:
- `--skip-details` (default: `True`, matching current hardcoded behavior) — when `False`, fetches body text via `_get_motion_details` → `_fetch_body_text`
- `--update-existing` (default: `False`) — when `True`, re-processes motions already in DB to fetch missing body_text and update the record

The update-existing flow:
1. Query motions table for rows WHERE date BETWEEN start_date AND end_date AND (body_text IS NULL OR body_text = '')
2. Extract besluit_id from the URL column (format: `https://www.tweedekamer.nl/kamerstukken/stemmingsuitslagen/{besluit_id}` — take last path segment)
3. For each such motion, call `api._get_motion_details(besluit_id)` to fetch body_text
4. UPDATE the motions row with the new body_text (and title/description if also missing)

Note: the motions table has no `besluit_id` column — it's only embedded in the URL. The update flow must parse it from the URL.

Run once after implementation: `--start-date 2016-01-01 --end-date 2018-12-31 --update-existing`
(No need for `--skip-details` when using `--update-existing` — it always fetches details for the targeted rows.)

### B. Bootstrap Computation (`analysis/political_axis.py`)

New function:
```
compute_party_bootstrap_cis(
    party_vectors: Dict[str, List[np.ndarray]],
    n_boot: int = 1000,
    ci: float = 95.0,
    seed: int = 42
) -> Dict[str, Dict]
```

Input: `party_vectors` is a dict mapping party name → list of individual MP vectors (each a numpy array of length 50). The caller (explorer.py) builds this from DB queries using existing mp→party mapping logic.

Returns per-party:
```
{
    "PVV": {
        "centroid": [50 floats],
        "ci_lower": [50 floats],
        "ci_upper": [50 floats],
        "std": [50 floats],
        "n_mps": 19
    },
    ...
}
```

Algorithm:
1. Receive pre-grouped `party_vectors` from caller
2. For each party with N >= 2:
   - Create numpy Generator with fixed seed
   - For each of n_boot replicates: sample N indices with replacement, compute mean vector
   - Compute percentile CIs (alpha/2, 100-alpha/2) and std across replicates per dimension
5. For parties with N = 1: set ci_lower == ci_upper == centroid, std = 0, flag n_mps = 1

Dependencies: numpy, duckdb (read_only), json.

**Import issue**: `_PARTY_NORMALIZE` and `CURRENT_PARLIAMENT_PARTIES` live in `explorer.py` (a Streamlit app). The bootstrap function in `analysis/political_axis.py` can't import from there. Solution: the bootstrap function accepts `party_vectors: Dict[str, List[np.ndarray]]` as input — the caller (explorer.py) handles the mp→party mapping and passes grouped vectors in. This keeps the analysis module independent of Streamlit app constants and avoids duplicating the normalization logic.

Alternatively, the caller can pass the already-computed `party_scores` dict from `load_party_axis_scores` plus raw per-party MP vector lists. The simplest approach: add a helper in explorer.py that loads grouped MP vectors per party (reusing existing mapping logic) and pass that to the bootstrap function.

### C. Chart Enhancement (`explorer.py`)

Modify `_render_party_axis_chart` to accept optional `bootstrap_data: Dict[str, Dict] = None`.

When bootstrap_data is provided:
- For each party, compute error magnitude: `(ci_upper[axis_idx] - ci_lower[axis_idx]) / 2`
- When flip is True, error magnitude stays the same (symmetric around the negated centroid)
- Add `error_x=dict(type="data", array=error_array, visible=True)` to the party marker Scatter trace
- Parties with N=1: render with a distinct marker (diamond shape instead of circle) as visual unreliability warning
- Add `N={n_mps}` to hover text for all parties

The bootstrap computation should be cached alongside party scores using `@st.cache_data`.

### D. Delete Stale JSON File

Remove `thoughts/explorer/party_svd_scores.json`. The app never reads this file — `load_party_axis_scores` always computes live from the DB. The file was generated ad-hoc during analysis and is missing NSC.

Also remove `thoughts/explorer/axis_analysis_data.json` — same situation, ad-hoc analysis artifact not used by the app.

## Data Flow

```
DB (svd_vectors, mp_metadata)
  │
  ├──→ load_party_axis_scores()
  │      returns Dict[str, List[float]]  (party → 50-dim centroid)
  │
  └──→ load_party_mp_vectors()  [NEW helper in explorer.py]
         returns Dict[str, List[np.ndarray]]  (party → list of individual MP vectors)
         reuses same mp→party mapping as load_party_axis_scores
  │
  ↓
compute_party_bootstrap_cis(party_vectors, n_boot=1000, ci=95, seed=42)
  │ returns Dict[str, Dict]  (party → {centroid, ci_lower, ci_upper, std, n_mps})
  ↓
_render_party_axis_chart(party_scores, comp_sel, theme, bootstrap_data=None)
  │ indexes [comp_sel - 1] from centroid and CIs
  │ applies flip (negate score AND CI bounds)
  │ adds error_x to Plotly Scatter trace
  ↓
Streamlit renders chart with error bars
```

Both functions cached via `@st.cache_data` with same TTL.

## Error Handling

- **N=1 parties (Volt, Lid Keijzer)**: Return centroid as both CI bounds, std=0. Chart renders diamond marker. Hover says "N=1, geen betrouwbaarheidsinterval".
- **N=2 parties (50PLUS)**: CIs will be wide — that's correct, let data speak.
- **SVD vector parsing failures**: Skip MP, log warning (same as existing pattern).
- **Download/scraping failures**: Per-chunk try/except already handles this. `_fetch_body_text` returns None on failure (existing behavior).
- **update-existing with no besluit_id**: Skip motion, log. Not all motions have a besluit_id traceable to body text.

## Testing Strategy

### Unit Tests
- `test_bootstrap_fixed_seed`: Synthetic data (5 parties, varying N), fixed seed. Verify:
  - Output shape matches expected structure
  - CI bounds bracket centroid for all parties
  - N=1 party has ci_lower == ci_upper == centroid
  - Same seed produces identical output
  - Larger N produces narrower CIs

### Integration Tests
- `test_bootstrap_real_db`: Run against actual DB, verify:
  - Returns data for all 17 current parliament parties (+NSC)
  - n_mps values match known party sizes
  - CI width for D66 (N=49) << CI width for SP (N=3)

### Visual Validation
- Run Streamlit app, verify error bars appear on SVD axis charts
- Verify N=1 parties have distinct marker style
- Verify hover text includes party size

## Open Questions

None — design is straightforward. The only future enhancement would be multi-window bootstrap for axis stability testing, but that's a separate project.
