Hook:  If your model “mysteriously” changes numbers, breaks relationships, or sorts months alphabetically—blame data types. What we’ll cover:  where to define types (and why), the safest order of operations across the ETL chain, Power Query vs. Model vs. Table View, special cases (DirectQuery, currency, duration), and concrete, copy-paste examples. 1) Why data types matter (and where they live) Power Query (PQ):  Data shaping stage. Types here affect folding, transformations