Excluding a response in pre and post comparison

[enorthviva]

Hi,

I have what is probably a very simple problem to solve.

I have a small dataset, which consists of responses to a questionnaire. The questionnaire is taken before and after an intervention, to see changes as a result of the intervention. 

In order to compare the "before" and "after" scenarios, I want to exclude those participants who completed a "before" questionnaire but not an "after" questionnaire. Participants each have a unique identifier.

Here is a snippet of the dataset:

You can see that in this part of the data, participant 9 (in the "assign number" column) has completed the pre- but not the post-questionnaire, while all others have completed both. How do I exclude participant 9 from the before and after comparisons that I am trying to do?

Thank you for your help!

[whitch]

Create this measure:

Exclude =

var aID = SELECTEDVALUE('Table'[What is the childs Assign Number])

return

CALCULATE(COUNTROWS('Table'),'Table'[Is this pre or post survey]="Pre",'Table'[What is the childs Assign Number]=aID)<1

Replace 'Table' with the name of your table and everything inside square brackets with whatever column names you actually have.

This measure evaluates to TRUE for the rows you want to exclude, so you can filter using this.