Why preprocessing belongs inside the experiment

Unit ID: ML-M07-U00 Estimated active time: 20-35 minutes

Why this matters

Preprocessing and Reproducible Pipelines helps learners avoid a common mistake: using a modelling technique without knowing what the result is allowed to mean.

In this unit, we focus on why preprocessing belongs inside the experiment because it is one piece of the larger workflow.

The idea

ROC-AUC and PR-AUC summarize ranking across thresholds, but they do not choose the operating threshold for you.

The classroom rule is simple:

Do the smallest honest experiment, then explain the limit.

For this topic, that means connecting the code or worksheet output to the decision context. A number, cluster, threshold, or model comparison is not useful until the learner can say what it means and what it does not mean.

Predict

Before running code, answer:

• What output do you expect?
• What could make that output misleading?
• What would you check before trusting it?

Run or inspect

Use the supplied synthetic dataset for Module 07. Read the column names first. Identify the target if the module has one. If there is no target, say clearly that the exercise is exploratory.

Keep the result small enough to inspect: a compact table, one metric summary, one cluster summary, or one threshold comparison.

Change one thing

Change one modelling choice or interpretation choice. For example, change a threshold, remove a risky feature, compare a baseline, scale the data, or inspect one subgroup.

Then ask:

Did the conclusion change, or only the number?

Practice

Build a pipeline with imputation, scaling, one-hot encoding, and a model.

Write a short note that names at least one limitation.

Check and explain

Complete this sentence:

This result suggests ______, but it does not prove ______.

Takeaway

Make transformations repeatable and prevent data preparation from contaminating evaluation. The model output is evidence to inspect, not a claim to overstate.