1. Item Response Theory in Health Measurement

2. Outline Contrast IRT with classical test theory
Introduce basic concepts in IRT
Illustrate IRT methods with ADL and IADL scales
Discuss empirical comparisons of IRT and CTT
Advantages and disadvantages of IRT
When would it be appropriate to use IRT?

3. Test Theory Any item in any health measure has two parameters:
The level of ability required to answer the question correctly. (In health this translates into the level of health at which the person doesn’t report this problem)
The level of discrimination of the item: how accurately it distinguishes well from sick
This means that estimates of item parameters (item-total correlation, difficulty, discrimination), will change with different types of samples. Must re-establish validity for a different population.
Total score is dependent on particular set of items used

4. Classical Test Theory
Most common paradigm for scale development and validation in health
Few theoretical assumptions, so broadly applicable
Partitions observed score into True Score + Error
Probability of a given item response is a function of person to whom item is administered and nature of item
Item difficulty: proportion of examinees who answer item correctly (in health: item severity…)
Item discrimination: biserial correlation between item and total test score.
This means that estimates of item parameters (item-total correlation, difficulty, discrimination), will change with different types of samples. Must re-establish validity for a different population.
Total score is dependent on particular set of items used

5. Classical test theory
Probability of ‘no’ answer depends on type of item (difficulty) and the level of physical functioning (e.g. SF-36 bathing vs. vigorous activities)
Some limitations
Item difficulty, discrimination, and ability are confounded
Sample dependent; item difficulty estimates will be different in different samples. Estimate of ability is item dependent
Difficult to compare scores across two different tests because not on same scale
Often, ordinal scale of measurement for test
Assumes equal errors of measurement at all levels of ability

6. Item Response Theory Complete theory of measurement and item selection
Theoretically, item characteristics are not sample dependent; estimates of ability are not item dependent
Item scores on same scale as ability
Puts all individual scores on standardized, interval level scale; easy to compare between tests and individuals
This means that item validity should be the same for different samples.
In CTT, total test score is dependent on difficulties of item. In IRT, item difficulty is considered in calculating total ability.
this means that estimate of ability should be the same for different groups of items.

7. Item Response Theory
Assumes that a normally distributed latent trait underlies performance on a measure
Assumes unidimensionality
All items measuring the same construct
Assumes local independence
Items are uncorrelated with each other when ability is held constant
Given unidimensionality, any reponse to an item is a monotonically increasing function of the latent trait (item characteristic curve)
Local independence: if you hold ability constant, the probality of reponding correctly to one item is independent of the probability of responding correctly to another item. This assumes that all variation in items is due to the underlying latent trait.

9. Example of item characteristic curves (Note the a parameter: 2
Example of item characteristic curves (Note the a parameter: 2.82 for the steep curve, 0.98 for the shallow curve)

12. Differential Item Functioning
Assuming that the measured ability is unidimensional and that the items measure the same ability, the item curve
should be unique except for random variations, irrespective of the
group for whom the item curve is plotted…
…items that do not yield the same item response function
for two or more groups are violating one of the fundamental
assumptions of item response theory, namely that the item and
the test in which it is contained are measuring the same
unidimensional trait…

13. Possible DIF

14. Item Bias
Items may be biased against one gender, linguistic, or social group
Can result in people being falsely identified with problems or missing problems
Two elements in bias detection
Statistical detection of Differential Item Functioning
Item review
If source of problems not related to performance, then item is biased

15. DIF detection Important part of test validation
Helps to ensure measurement equivalence
Scores on individual items are compared for two groups:
Reference
Focal group under study
Groups matched on total test score (ability)

16. DIF detection DIF can be uniform or nonuniform Uniform Nonuniform
Probability of correctly answering item correctly is consistently higher for one group
Nonuniform
Probability of correctly answering item is higher for one group at some points on the scale; perhaps lower at other points

17. Illustration of IRT with ADL and IADL Scales
The latent traits represent the ability to perform self-care activities and instrumental activities (necessary for independent living)
Item difficulty (b): the level of function corresponding to a 50% chance of endorsing the item
Item discrimination (a): slope of the item characteristic curve, or how well it differentiates low from high functioning people

18. 3 models
One-parameter (Rasch) model provides estimates of item difficulty only
Two-parameter model provides estimates of difficulty and discrimination
Three-parameter model allows for guessing
IRT does have different methods for dichotomous and polytomous item scales

19. IRT models: dichotomous items
One parameter model
Probability correct response (given theta) = 1/[1 + exp(theta – item difficulty)]
Two-parameter model
Probability correct response (given theta) = 1/{1 + exp [ – discrimination (theta – item difficulty)]}
Three parameter model:
Adds pseudo-guessing parameter
Two parameter model is most appropriate for epidemiological research

20. Steps in applying IRT Step One: Assess dimensionality Calibrate items
Factor analytic techniques
Exploratory factor analysis
Study ratio of first to second eigenvalues (should be 3:1 or 4:1)
Also χ2 tests for dimensionality
Calibrate items
Calculate item difficulty and discrimination and examine how well model fits
χ2 goodness of fit test
Compare goodness of fit between one-parameter and two-parameter models
Examine root mean square residual (values should be < 2.5)

21. Steps in IRT: continued
Score the examinees
Get item information estimates
Based on discrimination adjusted for ‘standard error’
Study test information
If choosing items from a larger pool, can discard items with low information, and retain items that give more information where it is needed

22. Item Information
Item information is a function of item difficulty and discrimination. It is high when item difficulty is close to the average level of function in the group and when ICC slope is steep

23. The ADL scale example
Caregiver ratings of ADL and IADL performance for 1686 people
1048 with dementia and 484 without dementia
1364 had complete ratings

24. ADL/IADL example Procedures
Assessed dimensionality. Found two dimensions: ADL and IADL
Assessed fit of one-parameter and two parameter model for each scale
Two-parameter better
Only 3 items fit one-parameter model
Sig. improvement in χ2 goodness of fit
Used two-parameter model to get item statistics for 7 ADL items and 7 IADL items

25. ADL/IADL
Got results for each item: difficulty, discrimination, fit to model
Results for item information and total scale information

26. Example of IRT with Relative’s Stress Scale
The latent trait (theta) represents the intensity of stress due to recent life events
Item severity or difficulty (b): the level of stress corresponding to a 50% chance of endorsing the item
Item discrimination (a): slope of the item characteristic curve, or how well it differentiates low from high stress cases
Item information is a function of both: high when (b) is close to group stress level and (a) is steep

27. Stress Scale: Item Information
item information is a function of item difficulty and discrimination. It is high when item difficulty is close to group stress level and when ICC slope is steep

28. Stress Scale: Item Difficulty
Item severity or difficulty (b) indicates the level of stress (on theta scale) corresponding to a 50% chance of endorsing the item

29. Stress Scale: Item Discrimination
item discrimination reflected in the slope of the item characteristic curve (ICC): how well does the item differentiate low from high stress cases?

30. Example of developing Index of Instrumental Support
Community Sample: CSHA-1
Needed baseline indicator of social support as it is important predictor of health
Concept: Availability and quality of instrumental support
Blended IRT and classical methods

31. Sample 8089 people Randomly divided into two samples: Procedures
Development and validation
Procedures
Item selection and coding
7 items

32. Procedure IRT analyses Tested dimensionality Two-parameter model
Estimated item parameters
Estimated item and test information
Scored individual levels of support

33. External validation Internal consistency Construct validity
Correlation with size of social network
Correlation with marital status
Correlation with gender
Predictive validity

34. Empirical comparison of IRT and CTT in scale validation
Few studies. So far, proponents of IRT assume it is better. However,
IRT and CTT often select the same items
High correlations between CTT and IRT difficulty and discrimination
Very high (0.93) correlations between CTT and IRT estimates of total score

35. Empirical comparisons (cont’d)
Little difference in criterion or predictive validity of IRT scores
IRT scores are only slightly better
When item discriminations are highly varied, IRT is better
IRT item parameters can be sample dependent
Need to establish validity on different samples, as in CTT

36. Advantages of IRT
Contribution of each item to precision of total test score can be assessed
Estimates precision of measurement at each level of ability and for each examinee
With large item pool, item and test information excellent for test-building to suit different purposes
Graphical illustrations are helpful
Can tailor test to needs: For example, can develop a criterion-referenced test that has most precision around the cut-off score

37. Advantages of IRT Interval level scoring
More analytic techniques can be used with the scale
Ability on different tests can be easily compared
Good for tests where a core of items is administered, but different groups get different subsets (e.g., cross-cultural testing, computer adapted testing)

38. Disadvantages of IRT Strict assumptions
Large sample size (minimum 200; 1000 for complex models)
More difficult to use than CTT: computer programs not readily available
Models are complex and difficult to understand

39. When should you use IRT? Cross-cultural testing In test-building with
Large item pool
Large number of subjects
Cross-cultural testing
To develop short versions of tests
(But also use CTT, and your knowledge of the test)
In test validation to supplement information from classical analyses

40. Software for IRT analyses
Rasch or one parameter models:
BICAL (Wright)
RASCH (Rossi)
RUMM
Two or three parameter models
NOHARM (McDonald)
LOGIST
TESTFACT
LISREL
MULTILOG