Measures
Measures, or in some earlier model versions, calculated fields, are without a doubt the most powerful element of Power BI models. In fact, most of the work we do on Power BI models comes down to designing and implementing DAX measures.
When using a numeric column from a fact table in a Power BI report, the column values will be aggregated. The basic aggregations available are sum, average, minimum, maximum, count, distinct count, and some statistical aggregations like standard deviation, variance, and median. The basic aggregations differ depending on the data type; for a Date column, for instance, you can choose earliest and latest, count, and distinct count only. When you use columns this way, the Power BI model creates an implicit measure in the background: an aggregation function that returns the selected aggregation of the values in the column.
In real life, many of the required insights come down to aggregations that cannot be expressed in terms of basic aggregations...
