It's a component of a proper measurement system analysis. Verifying the measurement fixtures ability to repeat and reproduce a measurement. We're obviously just "playing with toys" here so I would guess that you would simply want to fixture repeatedly the same spring and take measurements off of it. If your results vary by a ton you obviously have something wrong with the fixture. I've never checked spring rates before so I can't speak from experience but the results above showed what I would consider to be a LARGE variation that we would not want in our application on small vehicles.

I'm such a geek....