Difference Between Two Related Rows in a Table

Hi samdep ,

First, please make sure that what you are creating is a calculated column not a measure, then update its formula as below to get the expected result:

Judgment Variance = 
VAR _nextindex =
    MINX (
        FILTER (
            'Assessment',
            'Assessment'[Index] > EARLIER ( [Index] )
                && 'Assessment'[Manager & Direct Report] = 'Assessment'[Manager & Direct Report]
        ),
        [Index]
    )
VAR _nextrating =
    MAXX (
        FILTER (
            'Assessment',
            'Assessment'[Index] = _nextindex
                && 'Assessment'[Manager & Direct Report] = EARLIER ( 'Assessment'[Manager & Direct Report] )
        ),
        [Judgment]
    )
RETURN
    IF (
        ISBLANK ( _nextrating )
            || _nextrating = 'Assessment'[Judgment],
        BLANK (),
        ABS ( 'Assessment'[Judgment] - _nextrating )
    )

Best Regards