Model answers for 4 Power BI questions
Typical round: concepts, DAX on screen, a model design scenario
Q: What does CALCULATE do? A: It evaluates an expression in a modified filter context. For example, CALCULATE(SUM(Sales[Amount]), Region[Name] = "North") returns North sales regardless of the region slicer, unless other filters still apply. It is also how row context becomes filter context inside iterators.
Q: Calculated column vs measure? A: A calculated column is computed at refresh, stored per row and uses row context; use it for slicing or grouping. A measure is computed at query time in the current filter context; use it for aggregations like totals and ratios. Prefer measures to keep the model small.
Q: Why use a star schema? A: Fact tables hold numeric events and dimension tables hold descriptive attributes. Single-direction one-to-many relationships from dimensions to facts make filters predictable, DAX simpler and reports faster.
Q: A report is slow. What do you check? A: Use Performance Analyzer to find the slow visual, then check the model for unnecessary columns, high-cardinality fields, bidirectional relationships and heavy calculated columns, and simplify DAX that iterates large tables.