[Rasch] Permissible Transformations
Mike Linacre (RMT)
rmt at rasch.org
Thu Mar 19 14:56:09 EST 2009
Ratio scales: Most zero points (reference points) are chosen for
convenience. For instance,
when we make a ratio-scale statement such as "This mountain is twice as
high as that mountain", we have chosen sea-level as our zero-point;
when we make a ratio-scale statement such as "This room is twice as hot as
that room", we have chosen a room at a comfortable room-temperature as our
Ratio scales are not invariant against change of zero-point. If "John is
twice as tall as Jim" is true relative to the floor as zero-point, it is
not true relative to the center-of-the-Earth as zero-point.
Ratio scales are invariant against changes in interval-level
measurement-units. "John is twice as tall as Jim" is the same for both
centimeters and inches.
Hope this helps ....
Editor, Rasch Measurement Transactions
rmt at rasch.org www.rasch.org/rmt/ Latest RMT: 22:4 Spring 2009
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