# Take a stratified sample

Shape / rows

A variant of [Sample rows](https://meridian.online/operations/sample-rows).

Sample within each class of a column so the sample keeps, or evens out, the class shares.

Page: <https://meridian.online/operations/sample-rows/take-a-stratified-sample>

## Status

Runs as a SQL step.

arcform carries it as a `sql:` step today.

## The DuckDB step

sample within each class, keeping the class shares or evening them out

It reads the `orders` table the proofs share. Create it first:

```sql
CREATE TABLE orders AS SELECT * FROM (VALUES
  (1,  'Alice Ng',      'north', DATE '2026-01-03', 120.00, 'widget', 'a|b',   '{"channel":"web","items":[1,2]}'),
  (2,  'alice ng',      'north', DATE '2026-01-05',  80.50, 'gadget', 'b',     '{"channel":"shop","items":[3]}'),
  (3,  'Bob  Okafor',   'south', DATE '2026-01-05', 300.00, 'widget', 'a|c',   '{"channel":"web","items":[4,5,6]}'),
  (4,  'Carla Díaz',    'east',  DATE '2026-01-09',  NULL,  'gizmo',  NULL,    '{"channel":"web","items":[]}'),
  (5,  'Bob Okafor',    'south', DATE '2026-01-12',  45.25, 'gadget', 'c',     '{"channel":"phone","items":[7]}'),
  (6,  'Dev Patel',     'west',  DATE '2026-01-12', 510.00, 'widget', 'a',     '{"channel":"web","items":[8,9]}'),
  (7,  'Carla Diaz',    'east',  DATE '2026-01-20',  99.99, 'gizmo',  'b|c',   '{"channel":"shop","items":[10]}'),
  (8,  'Dev Patel',     'west',  DATE '2026-02-02', 510.00, 'widget', 'a',     '{"channel":"web","items":[8,9]}'),
  (9,  'Eve Larsen',    NULL,    DATE '2026-02-03',  15.00, 'gadget', NULL,    '{"channel":"web","items":[11]}'),
  (10, 'Alice Ng',      'north', DATE '2026-02-10', 220.00, 'gizmo',  'a|b|c', '{"channel":"phone","items":[12,13]}')
) t(order_id, customer, region, order_date, amount, product, tags, meta);
```

Then run the statement:

```sql
CREATE TEMP TABLE ranked AS
SELECT order_id, product,
       row_number() OVER (PARTITION BY product ORDER BY hash(order_id::VARCHAR || '-sample-7')) AS rn,
       count(*) OVER (PARTITION BY product) AS class_size
FROM orders;
-- keep a third of every class, rounded up: the shares stay close to 4 : 3 : 3
CREATE TEMP TABLE proportional AS SELECT order_id, product FROM ranked WHERE rn <= ceil(class_size / 3.0);
-- even the classes out: two rows from each
CREATE TEMP TABLE balanced AS SELECT order_id, product FROM ranked WHERE rn <= 2;
SELECT product, count(*) AS n FROM proportional GROUP BY product ORDER BY product;
SELECT product, count(*) AS n FROM balanced GROUP BY product ORDER BY product;
SELECT CASE WHEN ((SELECT map_from_entries(list((product, n) ORDER BY product)) FROM (SELECT product, count(*) AS n FROM proportional GROUP BY product)) = MAP {'gadget': 1, 'gizmo': 1, 'widget': 2} AND (SELECT list(order_id ORDER BY order_id) FROM proportional) = [1, 6, 7, 9] AND (SELECT list(order_id ORDER BY order_id) FROM balanced) = [1, 4, 5, 6, 7, 9] AND (SELECT map_from_entries(list((product, n) ORDER BY product)) FROM (SELECT product, count(*) AS n FROM balanced GROUP BY product)) = MAP {'gadget': 2, 'gizmo': 2, 'widget': 2}) THEN 'as expected' ELSE error('the proportional sample was not orders 1, 6, 7 and 9, or the balanced sample was not orders 1, 4, 5, 6, 7 and 9') END AS verdict;
```

It needs nothing installed beyond DuckDB itself.

## What other tools call it

Meridian's classification; an independent blind rating agreed on 95.7% of 324 rows (variant, version 3).

- KNIME Analytics Platform: [`Equal Size Sampling`](https://api.hub.knime.com/nodes/*nZoZFKRELGnk6y5i?details=full) (read 2026-09-27)
- RapidMiner Studio: [`Sample (Stratified)`](https://docs.rapidminer.com/latest/studio/operators/blending/examples/sampling/sample_stratified.html) (read 2026-09-27)