---
title: "SVD compass vs components tab party ordering inconsistency"
date: 2026-04-13
module: analysis
problem_type: ui_bug
component: analysis
symptoms:
  - "SVD components tab and political compass showed different party orderings for the same data"
  - "Party positions in compass did not match positions in SVD Components tab for components 1-2"
root_cause: logic_error
resolution_type: code_fix
severity: medium
tags:
  - svd
  - pca
  - compass
  - alignment
  - procrustes
---

# SVD Compass vs Components Tab Party Ordering Inconsistency

## Problem

The SVD Components tab and the political compass visualization showed different party orderings for the same data. Users would see a party at position X in the compass, but the same party at position Y in the SVD Components tab for components 1-2.

## Symptoms

- Same party (e.g., PVV) has different x-coordinate in compass vs SVD Components tab
- Party ordering along political axis differs between the two views
- Confusing user experience when exploring voting patterns

## What Didn't Work

Using raw SVD scores directly in the SVD Components tab. The compass uses Procrustes-aligned PCA positions from `load_positions()`, but components 1-2 in the SVD Components tab were using unaligned raw SVD scores. These are in different coordinate frames.

## Solution

For components 1-2 in the SVD Components tab, use aligned PCA positions from `load_positions()` (same data source as compass) instead of raw SVD scores. Components 3-10 continue to use raw SVD scores.

Added `_get_aligned_party_coords()` helper function in `explorer.py` that:
1. Calls `load_positions()` to get aligned MP positions
2. Aggregates MP positions to party centroids using `load_party_map()`
3. Returns `{party: (x, y)}` coordinates

```python
def _get_aligned_party_coords(window: str) -> Dict[str, Tuple[float, float]]:
    """Get party (x, y) coordinates from aligned PCA positions for a window."""
    positions_by_window, _ = load_positions(db_path, "annual")
    window_pos = positions_by_window.get(window, {})
    if not window_pos:
        return {}

    # Load party map to convert MP names to parties
    _party_map = load_party_map(db_path)

    # Aggregate MP positions to party centroids
    party_coords: Dict[str, List[Tuple[float, float]]] = {}
    for mp_name, (x, y) in window_pos.items():
        party = _party_map.get(
            mp_name, _party_map.get(mp_name.split("(")[0].strip(), None)
        )
        if party:
            party_coords.setdefault(party, []).append((x, y))

    # Compute mean position per party
    return {
        party: (
            float(np.mean([c[0] for c in coords])),
            float(np.mean([c[1] for c in coords])),
        )
        for party, coords in party_coords.items()
        if coords
    }
```

The rendering code now branches based on component:

```python
if comp_sel <= 2:
    # Components 1-2: use aligned PCA positions (consistent with compass)
    aligned_coords = _get_aligned_party_coords(svd_window)
    for party, (x, y) in aligned_coords.items():
        party_1d_coords[party] = (x,) if comp_sel == 1 else (y,)
else:
    # Components 3-10: use raw SVD scores
    idx = comp_sel - 1
    for party, scores in party_scores.items():
        if scores and len(scores) > idx:
            party_1d_coords[party] = (float(scores[idx]),)
```

## Why This Works

1. **Same coordinate frame**: Both visualizations now use Procrustes-aligned PCA positions for components 1-2
2. **Consistent party centroids**: Both aggregate MP positions to party centroids the same way
3. **Clear separation of concerns**: Components 1-2 represent political compass axes (need alignment), while components 3-10 are topic dimensions (use raw SVD scores)

## Prevention

- When adding new SVD/PCA visualizations, always check which data source the compass uses and use the same source for consistency
- Document coordinate frame requirements: "aligned" vs "raw" SVD scores have different interpretations
- Consider adding integration tests that verify compass and SVD Components tab show consistent positions

## Related Files

- `explorer.py` — `_get_aligned_party_coords()` helper, component 1-2 data loading
- `analysis/political_axis.py` — `load_positions()` and PCA alignment logic
- `analysis/explorer_data.py` — `load_party_scores_all_windows()` for components 3-10

## Related Issues

- This fix builds on the earlier SVD axis label alignment fix (`docs/solutions/ui-bugs/svd-axis-pole-labels-incorrect-after-flip.md`)