article 22740

Power BI Copilot, AI Instructions And Preventing The Use Of Implicit Measures

In yet another entry in my series on what you should be doing in Power BI Copilot AI Instructions, in this post I want to address the most difficult (in terms of deciding what to do, rather than how to do it) topic: whether you should allow the creation of implicit measures.

A quick recap of terminology: implicit measures are measures that are automatically created when you drag and drop a field into a visual and tell Power BI how the data should be aggregated (for example by summing, counting etc); explicit measures are measures that are specifically defined by a semantic model developer, have a name and have a DAX expression that specifies what data should be aggregated and how it should be aggregated. There is a property in a semantic model called “discourage implicit measures” that prevents report designers from creating implicit measures in the UI but it is by no means foolproof and at the time of writing Power BI Copilot does not respect it. This property was created for use with calculation groups but I’m going to leave the subject of Copilot and calculation groups for a future post.

Let’s see an example of an implicit measure created by Copilot. In the model I’ve been using in this series I have only two explicit measures, defined as follows:

Average Price Paid = AVERAGE('Transactions'[Price])
Count Of Transactions = COUNTROWS('Transactions')

The following prompt:

Show the number of distinct postcodes by month

Returns the following correct result by creating an implicit measure that does a distinct count on the Postcode column:

This is great – so why would you ever want to prevent this from happening and stop the use of implicit measures?

First of all, there’s always a danger with implicit measures that an end user – whether they are using Copilot or not – will try to aggregate data in a way that it should not be aggregated and therefore end up with incorrect or misleading results. For example the prompt:

Show the sum of month number by county

Returns a result, but not one that makes sense because you should never sum up values in the month number column:

[And this is despite the Summarization property of the Month Number column being set to “Don’t summarize”, which I need to talk to someone about]

Second, in complex semantic models, it may be necessary to include business logic in every explicit measure to ensure they always return the results you want. Implicit measures will not contain this logic and will therefore not return correct results.

As a result, if you’re sure you have created all the explicit measures an end user could ever need (and that’s definitely an achievable goal for many semantic models) then preventing the use of implicit measures could be a good thing.

Adding the following text to the model’s AI Instructions to prevent the use of implicit measures:

Never aggregate data using implicit measures. Only ever use the explicit measures defined in the semantic model to show aggregated data. If the user asks to aggregate data from a column, tell them that you aren't allowed to do this because of the risk of showing incorrect data.

Means that the following prompt from above:

Show the number of distinct postcodes by month

Now returns a message saying that Copilot isn’t allowed to aggregate data by itself:

As you can see, it is possible to use AI Instructions to prevent the use of implicit measures but this is something you really need to think hard about: it may help stop users getting incorrect results but it could also stop users from getting the correct results they need in some cases. It’s the old struggle between centralised, “the developer knows best” BI and decentralised self-service BI all over again.