1. Deborah Schnipke, PhD Virtual Psychometrics
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
Free demo of Rasch software

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