6 · Splitting, Validation & Experiment Design · Cross-Validation & Data Splitting

Stratification

Stratified splitting preserves class proportions. It is valuable for classification, especially when minority classes are small, but it does not solve subject or group dependence. The important practical question is not only how the technique is defined, but what assumptions it introduces, which data are allowed to influence it, and how its effect should be validated on unseen evidence.

Reference lessonPython exampleVisual explanation

Intuition first

What this concept means in practice

The practical value of Stratification comes from understanding both the transformation and the boundary around it: what information is allowed to enter, what assumption is being made, and how you know the result is still valid after the transformation.

A beginner-friendly way to reason about it is to start with a tiny case where the correct result can be checked independently. Once the mechanism is clear, scale the exact same reasoning to larger tables, pipelines or models.

PurposeUse for classification when class prevalence is uneven and samples are otherwise independent.

MechanismAllocate samples so each fold approximately preserves the class distribution of the full training set.

EvidenceInspect intermediate and final output; compare with an independent expectation.

Main cautionStratification preserves labels, not groups; repeated records from one person can still leak.

Mechanism

Trace the operation from input to decision

Allocate samples so each fold approximately preserves the class distribution of the full training set.

1Input→

2Apply rule→

3Inspect state→

4Validate→

5Use result

Key rule

class proportion per fold ≈ global proportion

Visual explanation

Make the structure visible

The interactive view uses a concept-specific plot when the topic maps naturally to one; otherwise it uses a workflow view instead of leaving a broken placeholder.

Loading visual…

Practical example

Where you would use it

With a 10% positive class, each fold should contain roughly 10% positives rather than allowing a fold with almost none.

Use when

Use for classification when class prevalence is uneven and samples are otherwise independent.

Pitfall

What can make the result misleading

Watch out

Stratification preserves labels, not groups; repeated records from one person can still leak.

A useful diagnostic question is: Could the same code still run successfully if the analytical assumption were wrong? If yes, add an explicit validation check rather than relying on execution success.

Implementation

Miniature Python example

Keep the example small enough that you can inspect each stage manually.

Python

# Purpose: demonstrate Stratification with a small, inspectable example.
# Follow the comments and printed stages to connect each operation with its result.
# Import the library or helper used in this example.
# Step 1 — Import the module so its functions/classes are available to the rest of this example.
import numpy as np
# Import the library or helper used in this example.
# Step 2 — Import only the named objects needed by the following steps, keeping dependencies explicit.
from sklearn.model_selection import StratifiedKFold

# Create the numerical values used in the calculation.
# Step 3 — Compute the right-hand expression and store its result in `X` for the next step.
X = np.arange(48).reshape(24,2)
# Step 4 — Construct `y` as an array so vectorised numerical operations can be applied consistently.
y = np.array([0]*12 + [1]*12)
# Store this intermediate value with a descriptive name for the next step.
# Step 5 — Compute the right-hand expression and store its result in `cv` for the next step.
cv = StratifiedKFold(n_splits=4, shuffle=True, random_state=7)
# Print this intermediate result so you can verify the workflow step by step.
# Step 6 — Display the current value explicitly so the result/state can be inspected during execution.
print("STEP 1 · Samples:", len(y), "class counts:", np.bincount(y).tolist())
# Iterate through the current values one item or step at a time.
# Step 7 — Iterate through the collection so the indented block is applied once for each item.
for fold,(tr,va) in enumerate(cv.split(X,y),1):
    # Print this intermediate result so you can verify the workflow step by step.
    # Step 8 — Display the current value explicitly so the result/state can be inspected during execution.
    print(f"STEP 2 · Fold {fold}: train={len(tr)} valid={len(va)} valid_classes={np.bincount(y[va]).tolist()}")
# Print this intermediate result so you can verify the workflow step by step.
# Step 9 — Display the current value explicitly so the result/state can be inspected during execution.
print("STEP 3 · Each sample is validation data once across the four folds.")

Expected / illustrative output

STEP 1 · Samples: 24 class counts: [12, 12]
STEP 2 · Fold 1: train=18 valid=6 valid_classes=[3, 3]
STEP 2 · Fold 2: train=18 valid=6 valid_classes=[3, 3]
STEP 2 · Fold 3: train=18 valid=6 valid_classes=[3, 3]
STEP 2 · Fold 4: train=18 valid=6 valid_classes=[3, 3]
STEP 3 · Each sample is validation data once across the four folds.

Implementation checklist

Before you move on

Can you state what data or object enters the operation?
Can you explain what changes and what must remain invariant?
Have you checked the result on a tiny case you can verify independently?
Have you considered the main failure mode described above?
Can the operation be reproduced from code/formulas and documented assumptions?