3.2. Create Model Calculations by Using DAX

💡 First Principle: A measure is a business definition written as code: "Revenue is the sum of extended amounts," "Margin % is margin divided by revenue," "Year-to-date resets every January." Written once in the model, it's evaluated in whatever filter context a visual provides. DAX's power, and its difficulty, comes from functions that change that filter context before calculating.

Why care? Measures are the product the business consumes. Executives never see your Power Query steps; they see numbers produced by measures, and they notice when last year's comparison is wrong or a balance is summed across months. DAX also carries heavy exam weight through hotspot items that ask you to complete an expression, so you need to predict results, not just recognize function names.

The mental model for the whole section is "change the filter, then compute." Simple aggregations compute over the current filter context. CALCULATE modifies the filter context first. Time intelligence functions are pre-built filter modifications over the date table. Calculation groups apply a filter modification to whichever measure is in play.

⚠️ Common Misconception: CALCULATE simply adds a filter on top of existing filters. A boolean filter argument such as Product[Color] = "Red" replaces any existing filter on Product[Color]. To intersect with what the user already selected, wrap it in KEEPFILTERS.