Featured Illustration
We've included Figure 3.1 to illustrate the principles that we used to
construct the developmental pathway variable maps in the book. There are some
important Rasch measurement ideas that are incorporated into the developmental
pathway illustrations.
You can take a look at the principles; look at the illustration and try to
answer the questions that follow to see if you've got the hang of it, if you
like.
Estimation
- Both items (as circles) and persons (as squares) are located on the same map.
- The logit scale is an interval scale, in which all logit units have the same
value.
- The highest values are located at the top of the map and the lowest values are
located at the bottom.
- Each item and person is located along the logit scale according to its
estimated value.
- More positive (higher) persons are more able and more positive (higher) items
are more difficult.
- The measurement error of the item and person estimates is indicated by the size
of the symbol: Larger symbols indicate greater error (in logits). Estimated
values are read vertically on the logit scale for both estimates and errors.
Fit
- Items and persons that fit the model's expectations are located in the white
zone.
- Items and persons that do not fit the model are located in the shaded zone.
- Fit values are read horizontally on a standardized t scale. Acceptable values
(white) fall between -2.0 and + 2.0.
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