Reader question — Testing data like you test code

Reader question on Testing data like you test code, and the answer belongs somewhere more findable than an email.

Your transformation logic is tested by unit tests. The data flowing through it is not, and that is where the failures actually happen.

The question was, roughly, "when does this stop applying?" — which is the right question and the one lessons routinely fail to answer. Every technique has a range of validity, and stating it is what separates a lesson from a recipe.

It sits in the Data quality and trust module. The exercise is a thinking one rather than a query one — some of this material is about deciding what to compute, and that does not need a keyboard.

Ordering matters here more than in most courses. It follows The patterns that keep coming up and leads into The anatomy of a silent failure, and reading it out of sequence mostly works but costs you the setup.

Free means free, and it also means we can rewrite it whenever it is wrong. No edition, no errata PDF, nothing to repurchase. The whole course is 36 lessons and the fixes land the day we find them.