# Rasch Analysis

Rasch analysis can be applied to assessments in a wide range of disciplines, including health studies, education, psychology, marketing, economics and social sciences.

Many assessments in these disciplines involve a well defined group of people responding to a set of items for assessment. Generally, the responses to the items are scored 0, 1 (for two ordered categories); or 0, 1, 2 (for three ordered categories); or 0, 1, 2, 3 (for four ordered categories) and so on, to indicate increasing levels of a response on some variable such as health status or academic achievement. These responses are then added across items to give each person a total score. This total score summarise the responses to all the items, and a person with a higher total score than another one is deemed to show more of the variable assessed. Summing the scores of the items to give a single score for a person implies that the items are intended to measure a single variable, often referred to as a unidimensional variable.

The Rasch model is the only item response theory (IRT) model in which the total score across items characterizes a person totally. It is also the simplest of such models having the minimum of parameters for the person (just one), and just one parameter corresponding to each category of an item. This item parameter is generically referred to as a threshold. There is just one in the case of a dichotomous item, two in the case of three ordered categories, and so on.

1.   What is Rasch Analysis

2.   Why undertake a Rasch analysis?

3.   The research paradigm and the Rasch model

4.   Is there more than one Rasch model?

5.   Different Rasch Model Specifications

6.   Thresholds and Steps

7.   Disordered Thresholds as an Anomaly

## 8. Who should use a Rasch analysis?

As implied from the above summary, a Rasch analysis should be undertaken by any researcher who wishes to use the total score on a test or questionnaire to summarize each person. There is an important contrast here between the Rasch model and Traditional or Classical Test Theory, which also uses the total score to characterize each person. In Traditional Test Theory the total score is simply asserted as the relevant statistic; in the Rasch model, it follows mathematically from the requirement of invariance of comparisons among persons and items.

A Rasch analysis provides evidence of anomalies with respect to

• the operation of any particular item which may over or under discriminate relative to the summary discrimination of all items.
• two or more groups in which any item might show differential item functioning (DIF).
• anomalies with respect to the statistical independence of the items.

If the anomalies do not threaten the validity of the Rasch model or the measurement of the construct, then

• people can be located on the same linear scale as the items.
• the locations of the thresholds of the items on the continuum permits a better understanding of the variable at different parts of the scale.
• the locations of the thresholds of the items on the continuum permits a better understanding of the variable at different parts of the scale.

The aim of a Rasch analysis is analogous to helping construct a ruler, but with the data of a test or questionnaire.

9.   An ideal approach to a Rasch analysis?

10. Recommended Rasch Software

11. The RUMM approach to Rasch Analysis

12. What courses and workshops are available on Rasch analysis?

13. What cloud analysis engines and API's are available?

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