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Rasch vs. IRT

Rasch vs. IRT. Deborah Schnipke, PhD Virtual Psychometrics. Why this talk?. Make people aware of the controversy Present the Rasch point to view Clear up misconceptions. Disclaimers. Put this talk together today I’m used to Rasch vs. 3PL

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Rasch vs. IRT

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  1. Rasch vs. IRT Deborah Schnipke, PhD Virtual Psychometrics

  2. Why this talk? • Make people aware of the controversy • Present the Rasch point to view • Clear up misconceptions

  3. Disclaimers • Put this talk together today • I’m used to Rasch vs. 3PL • Difference is more extreme (the 3rd parameter really messes up measurement properties, compared to Rasch) • Speak Ohio-style (even faster than New York)

  4. The Controversy • Different Philosophy/Different Point of View • Difference is very esoteric/theoretical • Hard to make this point in an article that is about something else • Raschies: • Very convinced of their position (can be hard to talk to) • Use different notation (can be confusing) • Small minority • Larger following in Europe historically, but this is changing (IRT becoming more common in Europe too) • IRTers: • Very convinced of their position • Majority of researchers/practitioners (in US, except for Chicago)

  5. Rasch ≠ 1PL IRT • Same model, different mindset • Theory based, rather than data based • Different set of diagnostic tools (different way of viewing data/results) • Item maps • Fit statistics that work

  6. IRT Mindset • Want to measure the items as well as possible • Use as many parameters as needed

  7. Rasch Mindset • Want to create a measurement scale that has interval-level properties (not “more interval-level”) • Use the model that leads to desirable measurement properties • Adding discrimination parameter breaks some of the scale properties

  8. Misconceptions • Only reason to use Rasch is that the math is easier • Rasch is not as good because it doesn’t have as many parameters • Rasch is only a small step away from raw scores

  9. Body Size Example • IRT approach: measure everything that seems relevant • E.g., weight, height, density, water displacement, etc • Rasch approach: define exactly what you mean by size & measure that (1 variable)

  10. Orange Juice Example Example: want to make 16oz juice from oranges • Classical test theory: count the oranges • IRT: describe oranges as well as possible & summarize with theta • Size/diameter, juiciness, weight • Rasch: measure what will lead to results you want (weight of oranges)

  11. Which is Better? (OJ example) • Raw scores: number of oranges • Rasch: weight of oranges • IRT: size? of oranges (juice-producing-ness?) • To make 16 oz of juice, need • 5-12 oranges, usually, maybe more, maybe less • Classical test theory (raw score) approach • 4 lbs of oranges • Rasch: direct interpretation • IRT: not quite sure what the interpretation is • Scoring is done by an algorithm

  12. Clinical Example • Paper: “Use of Rasch person-item map in exploratory data analysis: A clinical perspective” from Journal of Rehabilitation Research & Development • Study of Visual Impairment • Rate 48 daily activities (“not difficult” to “impossible”) • See item map & fit statistics (on website)

  13. Links • Clinical example found at http://www.vard.org/jour/04/41/2/stelmack.html • Free demo of Rasch software www.winsteps.com/ministep.htm

  14. Summary • Differences between Rasch & IRT are usually small • Rasch: probably better theoretically • IRT: easier to get published in journals • More people are IRTers & they are the reviewers/editors • Both are much better than raw scores • Small samples: use Rasch

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