DAX (Data Analysis Expressions) is a formula language introduced by Microsoft to interact with data in a variety of their platforms like Power BI, Analysis Services, and Power Pivot in Excel.

Below I'm providing an example of creating a row context within a measure.

Code

// Defining a measure
MyMeasure :=
CALCULATE (
    // Aggregate function here. (Eg: SUM, AVERAGE, COUNT, etc.)
    SUM ( 'YourTable'[YourColumn] ), 

    // Here we create a row context using the FILTER function
    FILTER (
        'YourTable',
        'YourTable'[YourColumn] > AVERAGE( 'YourTable'[YourColumn] )
    )
)

In this measure, CALCULATE function changes the context in which the data is evaluated by applying a filter to the data. The FILTER function is being used to create a row context, i.e., the filter is applied to each row of 'YourTable'.

How to use

To use the measure, you simply need to drag it into a table, chart, matrix, etc. in your Power BI report or slice it by another column from your model.

Unit Test

To perform a unit test on the created measure, you can create a PivotTable in the PowerPivot window, drag the measure into values area, and validate the output for correctness.

For expanding your knowledge about DAX or any other related topic, I recommend checking out related courses on Enterprise DNA Platform, it provides a wide array of courses that cater to your need for learning in-depth concepts and applications of DAX.

Hope this helps! Please let me know if you have any questions!