Reader question — Rows, grain, and the shape of a dataset

Reader question on Rows, grain, and the shape of a dataset, and the answer belongs somewhere more findable than an email.

Nearly every wrong number in a career traces back to someone not knowing what one row of their table represents.

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 Foundations module, and the exercise runs against retail-orders. 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 What a data team is actually for and leads into Turning a request into a question, 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.