[Rasch] Why are Rasch measures linear?

Anthony James luckyantonio2003 at yahoo.com
Mon Sep 3 06:41:56 EST 2007

Dear all,
  I have a basic question on the Rasch model.
  We always hear that Rasch measures are linear while raw scores aren't. Linearity is defined as "equal increments to the difficulty or ability measures when one more unit is added to a person's or an item's location".
  What I can't grasp is why Rasch measures are linear? What's magic about the Rasch measures. Once I heard  "Rasch measures are cnstructed to be linear". But how?
  I'd be grateful if someone explains in laymen terms that why Rasch measures are linear.

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