AI & Data

Its usefulness depends on relevance, measurement quality, permissions, and coverage of the intended task. More records do not automatically correct systematic errors or missing populations.

Key takeaways

• Define the unit of an example.
• Use only information available at prediction time.
• Track data provenance, missingness, and subgroup coverage.

Start by defining what one example represents. A row might describe a customer, a transaction, a photograph, or one moment in a time series. Those units determine how duplicates, labels, and evaluation splits should work. Ten measurements from one device are not necessarily ten independent devices. Features are inputs available to the model. Labels are target outcomes used in supervised learning. Check when each feature becomes available: a cancellation reason recorded after a customer leaves cannot fairly predict that departure beforehand. This is a form of leakage even when the field looks highly predictive. Inspect missing values, annotation disagreements, unusual ranges, and changes in collection methods. Missing information can carry meaning; replacing every missing value with zero can conflate an unknown quantity with a real zero. Document the treatment and test it on representative examples. Record provenance and access rules alongside the dataset. A public URL alone does not establish permission to reuse every item for every purpose. Collect only information needed for the task and define retention and deletion procedures. Evaluate separately on groups or conditions where errors would otherwise disappear inside an overall average.

1. Imagine records with signup date, monthly usage, cancellation date, and cancellation reason.
2. To predict cancellations at the start of June, freeze every input at that date. Remove reasons and dates recorded after the prediction time.
3. Train on earlier periods and test on a later untouched period. Compare results with and without the leaked fields.

This hypothetical design exercise identifies an invalid shortcut before a flattering score becomes a deployment decision.