DAX - Blank Data

Hi spandy34

For a calculated column, the values returned should be in the same data type so your expected result cannot be achieved. Currently "[Current Performance]-[Previous Performance]" will return a number while " No Previous Data" is in Text type. Returning different types of values in a calculated column will give you the following variant-type error. 

Please consider a different result when the Previous performance is blank. Or you can always convert "[Current Performance]-[Previous Performance]" result to Text type with CONVERT function. 

Difference =
IF (
    ISBLANK ( [Previous Performance] ),
    "No Previous Data",
    CONVERT ( [Current Performance] - [Previous Performance], STRING )
)

Best Regards,
Community Support Team _ Jing
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