Reader question — The first hour with an unfamiliar dataset

Reader question on The first hour with an unfamiliar dataset, and the answer belongs somewhere more findable than an email.

A repeatable routine for finding out what you are actually holding, before you compute anything anyone will act on.

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, and the exercise runs against dirty-customers. That pairing is deliberate: the dataset was built with the trap the lesson describes already in it, so the exercise fails in the instructive way rather than the confusing one.

Ordering matters here more than in most courses. It follows The anatomy of a silent failure and leads into Describing a column without lying, 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.