DAX Function to Compute Sum for First 4 Ranks

This document outlines a DAX function that computes the sum of costs for the top 4 items ranked by their cost from a Goods table. It utilizes the RANKX function to determine the ranking of each good based on the Cost column and ensures that calculations reflect correct grand totals.

Function Overview

Purpose

• To calculate the sum of costs for the first 4 ranked items based on their cost.

Parameters

• Table: A table named Goods containing at least two columns: Good (item names) and Cost (the cost of the items).

Return Type

• Returns a numeric value representing the total cost of the top 4 ranked items.

Exceptions

• Raises an error if the Goods table is not valid or if the Cost column does not exist.

DAX Code

Top4RankedSum = 
VAR RankedGoods =
    ADDCOLUMNS(
        Goods,
        "Rank", RANKX(Goods, [Cost], , DESC, DENSE)
    )
VAR Top4Goods =
    FILTER(RankedGoods, [Rank] <= 4)
RETURN
    SUMX(Top4Goods, [Cost])

Explanation of the Code

• ADDCOLUMNS Function: This is used to create a new column "Rank" in the RankedGoods variable that ranks the items in the Goods table based on their Cost.
• RANKX Function: It calculates the rank for each item, ordering by [Cost] in descending order. The DENSE option ensures ranks are continuous (1, 2, 2, 3).
• FILTER Function: It filters the ranked items, keeping only those with a rank of 4 or less.
• SUMX Function: It computes the total cost for the filtered results.

Grand Total Handling

This function will automatically handle grand totals accurately in reports, summing all ranks as appropriate while showing only the sum of the top 4 ranked items when the context is limited to specific categories or groups.

Example Usage

To use this function in a Power BI report, you can create a measure using the DAX expression provided. Then, you can place this measure in your report visuals to display the total cost of the top 4 goods based on their rank.

Sample Measure Usage

Assuming you want to show this in a card visual:

1. Go to the "Modeling" tab.
2. Click on "New Measure."
3. Copy and paste the Top4RankedSum DAX code.
4. Use the created measure in a card visual to display the sum.

Conclusion

This DAX function effectively computes the sum of a specified number of top-ranked items, ensuring that grand totals are calculated correctly. Users can apply this function in various reporting scenarios within Power BI to focus on high-value goods.

For further mastery of DAX and analytics, consider exploring the Enterprise DNA Platform for advanced courses and resources.