Comparing different scales

Hello Everyone,

I've completed an experiment on approx 70 subjects. For each subject I
have two scores: ROSE (= Explicit self-esteem) and ISE (= implicit self
esteem). I want to quantify the gap, or difference, between these
scores within each subject.

The problem is that the scores use different scales. ROSE is a grade
between 1 and 7 (the higher the grade,the higher the explicit self
esteem). ISE is a time measurement approx. between -3000 and 3000msec
(the higher the score, the higher the implicit self esteem).

How can I compute a new variable that will give me "ROSE - ISE"? How
can I meaningfully subtract one score from the other when they are on
these different scales?

I've tried converting each variable into z-scores and examining that,
but I suspect the process has drastically reduced the difference inside
each score and the resulting subtraction is useless. Am I correct? Is
there a better way to do this?

Thanks in advance,
Uri.

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