slicemap

Find the data slices where a new model regressed against an old one. Headless, file in, table out.

A new model can lift the overall metric while quietly getting worse on a segment that matters: one country, one age band, one product category. An aggregate number hides it. slicemap takes a predictions file with both models' outputs and the features, scores every slice, and lists the ones where the new model lost ground, ranked by how many rows are affected.

$ slicemap compare preds.parquet --true label --old pred_v1 --new pred_v2
slicemap: accuracy overall 0.910 -> 0.918
feature   slice        size   old     new    regression
country   BR            842   0.904   0.731   -0.173
device    tablet        311   0.880   0.795   -0.085
age       [55, 70)      540   0.901   0.860   -0.041

Install

$ pip install slicemap-cli                 # from PyPI, once released
$ pip install git+https://github.com/jmweb-org/slicemap   # latest, available now

Reads one CSV, Parquet or JSON Lines file containing the truth column, both prediction columns, and the feature columns.

Usage

$ slicemap compare preds.parquet --true y --old pred_a --new pred_b
$ slicemap compare preds.csv --true y --old a --new b --features country,age
$ slicemap compare preds.csv --true y --old a --new b --metric error
$ slicemap compare preds.csv --true y --old a --new b --min-slice 50
$ slicemap compare preds.csv --true y --old a --new b --json
$ slicemap compare preds.csv --true y --old a --new b --check

If --features is omitted, every column except the truth and prediction columns is treated as a feature.

JSON output schema

--json writes a single object to stdout:

{
  "metric": "accuracy",
  "old_overall": 0.910,
  "new_overall": 0.918,
  "regressions": [
    {
      "feature": "country",
      "slice": "BR",
      "size": 842,
      "old_score": 0.904,
      "new_score": 0.731,
      "regression": 0.173,
      "impact": 145.766
    }
  ]
}

Field	Type	Description
metric	string	Metric name used for scoring (e.g. "accuracy")
old_overall	number	Overall metric score for the old model
new_overall	number	Overall metric score for the new model
regressions	array	Slices where the new model is worse, sorted by impact descending
regressions[].feature	string	Column name the slice is drawn from
regressions[].slice	string	Slice label (a category value or a quantile bin like "[55, 70)")
regressions[].size	integer	Number of rows in the slice
regressions[].old_score	number	Old model's metric score on this slice
regressions[].new_score	number	New model's metric score on this slice
regressions[].regression	number	Absolute degradation (always positive)
regressions[].impact	number	regression × size — used for ranking

All numeric values are rounded to six decimal places.

In CI

Fail a model update when any slice regresses:

- run: slicemap compare preds.parquet --true y --old champion --new challenger --check

How slicing works

Categorical features slice by value; numeric features slice by quantile bins. For each slice the metric is computed for both models, and the slice is flagged when the new model is worse. Slices smaller than --min-slice are skipped to avoid noise, and findings are ranked by impact (regression size times the number of rows), so the segments worth fixing first come first.

Metrics

Metric	Direction
accuracy	higher is better
error	lower is better
mae	lower is better

Exit codes

Code	Meaning
0	Compared; no slice regressed (or --check not set)
1	--check found at least one regressed slice
2	A column was missing, the metric is unknown, or the file is unsupported

License

MIT. See LICENSE.