Why is the data model more important than the visualizations?

New Power BI users spend most of their time on appearance. Experienced users spend most of their time on the model. That is no coincidence.

Incorrect relationships produce incorrect figures — without an error message

If the relationships between the tables are set up incorrectly, the report will still display figures. They will simply be wrong. This is the most dangerous source of errors in Power BI, because there are no warnings and the charts look just as convincing.

A date table is almost always necessary

If you want to compare periods, show year-to-date figures, or calculate growth, the model needs a dedicated date table marked as such. Without it, time calculations will either be incorrect or impossible.

Measures rather than calculations in the visualization

Add calculations to the model as measures, not to individual visualizations. This ensures that the same definition applies everywhere, and the next person who builds a report inherits it instead of inventing their own.

The impact on the project plan

Expect the modeling work to take up most of the time. A project planned around the number of reports to be created will almost always underestimate the amount of work involved.