Comprehensive Power BI DAX best practices and patterns based on Microsoft...

# Power BI DAX Best Practices

## Overview
This document provides comprehensive instructions for writing efficient, maintainable, and performant DAX (Data Analysis Expressions) formulas in Power BI, based on Microsoft's official guidance and best practices.

## Core DAX Principles

### 1. Formula Structure and Variables
Always use variables to improve performance, readability, and debugging:

```dax
// ✅ PREFERRED: Using variables for clarity and performance
Sales YoY Growth % =
VAR CurrentSales = [Total Sales]
VAR PreviousYearSales = 
    CALCULATE(
        [Total Sales],
        SAMEPERIODLASTYEAR('Date'[Date])
    )
RETURN
    DIVIDE(CurrentSales - PreviousYearSales, PreviousYearSales)

// ❌ AVOID: Repeated calculations without variables  
Sales YoY Growth % =
DIVIDE(