IRT
Lower Asymptome
Guessing – Lower Asymptote
The inclusion of a “gamma” parameter suggests that respondents very low on the trait may still choose the correct answer
In other words, respondents with low trait levels may still have a small probability of endorsing an item
This is mostly used with multiple choice testing
Upper Asymptote
“Carelessness”/response style – Upper Asymptote
The inclusion of a “d” parameter suggests that respondents very high on the latent trait are not guaranteed (i.e., have less than 1 probability) to endorse the item
Often an item that is difficult to endorse (e.g., suicide ideation as an indicator of depression)
Carelessness
Invariance - IRT model parameters have an invariance property
graphic
• Item difficulty parameters do not depend on a particular population of examinees (see figure on the next slide) if the IRT model holds (testable assumption)
Reference groups
In CTT scores have only meaning relative to the persons in a sample, and thus sample norms are needed to interpret a person’s score
I got a score of 15. Is that good?”
“That puts you in to the 90th percentile of the sample of undergraduates” “Great!”
“That puts you into the 10th percentile of the sample of graduates” “WHAT?”
=> Same score in both cases but different reference group!
In IRT, the properties of items and persons are placed along the same underlying latent continuum, the joint scale of item difficulties and person abilities
=> This concept can be illustrated using construct maps that order both persons in terms of ability and items in terms of difficulty...
Advantages of IRT
3
IRT vs. CTT
Which one is superior?
Last changed3 months ago