## Paired comparisons with ties: Bradley-Terry and Rasch

The Bradley-Terry model for paired comparisons was formulated as a descriptive model, but can be written as a Rasch measurement model (Rasch RMT 9:2, 424):

loge (Pn>m/Pn<m) = Bn - Bm

where Pn>m is the probability that n is preferred to m. Bn and Bm are the desirability of n and m. This model does not allow for ties. Davidson and Beaver (D&B, 1977) propose an extension to the Bradley-Terry model that allows for ties by means of a parameter v, so that

Pn=m = v * sqrt(Pn>m * Pn<m)

D&B's model, however, does not permit separation of the parameters, and so it does not provide the interpretative power or inferential stability of a Rasch measurement model. Nevertheless, Matthews and Morris (M&M, 1995) apply D&B's model to their paired-comparison of the pain-alleviating effects of 4 local anaesthetic creams: 2 with active ingredients and 2 look- alike placebos (see Table).

```Applied      Applied Second
First      B-active B-placebo A-active A-placebo
B-active      -     4,3,0     6,0,1*   8,0,0
B-placebo  0,4,3      -       4,2,2    7,0,1*
A-active   0,0,7    1,0,7       -      5,1,1
A-placebo  1*,0,7   0,0,7     2,3,2      -
```

Numbers of patients record in the order: preferring first, no preference, preferring second. * are the three very unexpected observations.

In M&M's experiment, one cream had to be applied first. This gives that cream an advantage, analogous to that of the home team at a sports event. A Rasch model for paired comparisons with ties and an order effect (for which cream is applied first) is:

loge(Pn>m/Pn=m) = Bn + F - Bm - T
loge(Pn=m/Pn<m) = Bn + F - Bm + T

where F parameterizes the advantage of being treated first, and T parameterizes the tie, "no preference", zone. The lack of subscripts indicates that F and T are regarded as constant across all comparisons.

The measurement system resulting from a Facets analysis is informative (see Figure). Active creams are always preferred to their placebo look-alikes. B-Active cream is most preferred. A- active is not preferred even as much as B-placebo. Since the order effect (difference between First and Second use) is less than the difference between any pair of creams, the differences between the creams has a general import.

```-----------------------------------
|   Measure   |  Creams   | Order |
-----------------------------------
|      2      + B-Active  +       |
| (preferred) |           |       |
|             |           |       |
|             |           |       |
|             |           |       |
|             |           |       |
|             |           |       |
|             |           |       |
|             |           |       |
|             | B-placebo |       |
|      1      +           +       |
|             |           |       |
|             |           |       |
|             |           |       |
|             |           |       |
|             |           |       |
|             |           |       |
|             |           |       |
|             | A-active  |       |
|(not prefer.)|           | First |
|      0      * A-placebo * Second|
-----------------------------------
```

There are three very unexpected preferences. The most unexpected response is in the last data row. It is by the patient who preferred A-placebo over B-active (see Table), indicating how powerful a force the mind can be in pain-control!

The fact that, in general, each active cream was preferred to its placebo version, together with the generally consistent pattern of responses, reassures the analyst about both the quality of the data collection and the reasonableness of the analysis.

Davidson RR, Beaver RJ (1977) On extending the Bradley-Terry model to incorporate within-pair order effects. Biometrics, 33, 693-702.

Matthews JNS, Morris KP (1995) An application of Bradley-Terry- type models to the measurement of pain. Applied Statistics, 44(2) 243-255.

Paired comparisons with ties: Bradley-Terry and Rasch. Linacre JM. … Rasch Measurement Transactions, 1995, 9:2 p.425

Rasch-Related Resources: Rasch Measurement YouTube Channel
Rasch Measurement Transactions & Rasch Measurement research papers - free An Introduction to the Rasch Model with Examples in R (eRm, etc.), Debelak, Strobl, Zeigenfuse Rasch Measurement Theory Analysis in R, Wind, Hua Applying the Rasch Model in Social Sciences Using R, Lamprianou El modelo métrico de Rasch: Fundamentación, implementación e interpretación de la medida en ciencias sociales (Spanish Edition), Manuel González-Montesinos M.
Rasch Models: Foundations, Recent Developments, and Applications, Fischer & Molenaar Probabilistic Models for Some Intelligence and Attainment Tests, Georg Rasch Rasch Models for Measurement, David Andrich Constructing Measures, Mark Wilson Best Test Design - free, Wright & Stone
Rating Scale Analysis - free, Wright & Masters
Virtual Standard Setting: Setting Cut Scores, Charalambos Kollias Diseño de Mejores Pruebas - free, Spanish Best Test Design A Course in Rasch Measurement Theory, Andrich, Marais Rasch Models in Health, Christensen, Kreiner, Mesba Multivariate and Mixture Distribution Rasch Models, von Davier, Carstensen
Rasch Books and Publications: Winsteps and Facets
Applying the Rasch Model (Winsteps, Facets) 4th Ed., Bond, Yan, Heene Advances in Rasch Analyses in the Human Sciences (Winsteps, Facets) 1st Ed., Boone, Staver Advances in Applications of Rasch Measurement in Science Education, X. Liu & W. J. Boone Rasch Analysis in the Human Sciences (Winsteps) Boone, Staver, Yale Appliquer le modèle de Rasch: Défis et pistes de solution (Winsteps) E. Dionne, S. Béland
Introduction to Many-Facet Rasch Measurement (Facets), Thomas Eckes Rasch Models for Solving Measurement Problems (Facets), George Engelhard, Jr. & Jue Wang Statistical Analyses for Language Testers (Facets), Rita Green Invariant Measurement with Raters and Rating Scales: Rasch Models for Rater-Mediated Assessments (Facets), George Engelhard, Jr. & Stefanie Wind Aplicação do Modelo de Rasch (Português), de Bond, Trevor G., Fox, Christine M
Exploring Rating Scale Functioning for Survey Research (R, Facets), Stefanie Wind Rasch Measurement: Applications, Khine Winsteps Tutorials - free
Facets Tutorials - free
Many-Facet Rasch Measurement (Facets) - free, J.M. Linacre Fairness, Justice and Language Assessment (Winsteps, Facets), McNamara, Knoch, Fan

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