Your Research Tips: Rasch Model: Simply Easy to Make a Measurement & Assessment

Rasch Model: Simply Easy to Make a Measurement & Assessment!

Rasch Model 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.

What is Rasch Model?
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.

Working in Quantitative Data Analysis? So What?
The Rasch model, where the total score summarizes completely a person's standing on a variable, arises from a more fundamental requirement: that the comparison of two people is independent of which items may be used within the set of items assessing the same variable. Thus the Rasch model is taken as a criterion for the structure of the responses, rather than a mere statistical description of the responses.
For example, the comparison of the performance of two students' work marked by different graders should be independent of the graders. In this case it is considered that the researcher is deliberately developing items that are valid for the purpose and that meet the Rasch requirements of invariance of comparisons.

Analyzing data according to the Rasch model, that is, conducting a Rasch analysis, gives a range of details for checking whether or not adding the scores is justified in the data. This is called the test of fit between the data and the model. If the invariance of responses across different groups of people does not hold, then taking the total score to characterize a person is not justified. Of course, data never fit the model perfectly, and it is important to consider the fit of data to the model with respect to the uses to be made of the total scores.

If the data do fit the model adequately for the purpose, then the Rasch analysis also linearises the total score, which is bounded by 0 and the maximum score on the items, into measurements. The linearised value is the location of the person on the unidimensional continuum - the value is called a parameter in the model and there can be only one number in a unidimensional framework. This parameter can then be used in analysis of variance and regression more readily than the raw total score which has floor and ceiling effects.

Why undertake a Rasch analysis?

•   A researcher who is developing items of a test or questionnaire intending to sum the scores on the items can use a Rasch model analysis to check the degree to which this scoring and summing is defensible in the data collected. For example, if two groups are to be compared on the variable of interest (e.g. males and females), it is important to demonstrate that the workings of the items is the same in the two groups. Working in the same way permits interpreting the total score as meaning the same in the two groups.
•   In checking how well the data fit the model, it is important to be able to diagnose very quickly where the misfit is the worst, and then proceed to try to understand this misfit in terms of the construction of the items and the understanding of the variable in terms of its theoretical development.
•   A very important part of the Rasch analysis from this perspective is to be in dynamic and interactive control of an analysis and to be able to follow the evidence to see where the responses may be invalid.

(Sources: http://www.rasch-analysis.com/rasch-measurement.htm)

Our next workshop. Don't miss it!

Date	»	19 & 20 May 2014 (Monday - Tuesday)
Time	»	8.30 a.m. - 5.30 p.m.
Venue	»	MPWS Training Centre, 63-1, 63-2, Jalan Kajang Impian 1/11, Taman Kajang Impian, Seksyen 7, 43650 Bandar Baru Bangi, Selangor # map
Speaker	»	Dr. Akbariah Binti Mohd Mahdzir
Registration Fee	»	RM300 (early bird rate) / RM400 (normal rate)
Medium	»	English
Website	»	http://postgraduateworkshop.com/raschanalysis

Introductory Rasch Model Workshop: Applications in Survey Research and Educational Measurement

The research paradigm and the Rasch model

There is a philosophical or paradigm difference between the application of the Rasch model and other IRT models, for example, the two-parameter and three-parameter models are designed for responses scored just 0, 1.. In the paradigm of other IRT models, the emphasis is on finding a model that best characterizes the given data; in the Rasch paradigm, the emphasis is in identifying and studying anomalies in the data disclosed by the Rasch model. Thus, in the paradigm of applying other than Rasch models, if the Rasch model does not work then a more complicated model, relative to the simpler Rasch model, that might explain the data better is sought. In the case of dichotomously scored data, this might be the two parameter model which has a second parameter for each item. 

Is there more than one Rasch model?

There is only one Rasch model for unidimensional responses at the level of one person responding to one item.However, there are different specifications when more than two ordered response categories are present. In one specification, all items might be hypothesized to have the same parameters across all items, as for example in the case that all items have the same response structure (e.g. SD, D, A, SA). In a second specification, different parameters across items may be needed when items do not have the same response categories, as in achievement testing when different items may have a different number of ordered categories and most certainly a different description of the categories 

Different Rasch Model Specifications

For the case where the response categories are the same across items (e.g. SD, D, A, SA), the Rasch model has been called "the rating scale model"; the case where the response categories are different across items has been called the "partial credit model". It is stressed, however, that the structure and response process for a person responding to an item is identical in the two specifications. Rather than emphasizing two models for the above different specifications, it can be more efficient to refer to one RaschUnidimensional Measurement Model (RUMM) with different numbers of categories and different parameterizations (as in RUMM2030). Thus it might be better to refer to the former as a rating scale parameterization; the latter as a partial credit parameterization. 

Thresholds and Steps

One particular difference that has arisen in different Rasch analysis reporting is the use of "step", when the parameters are different across items, and "thresholds", when they are the same across items. This can give the impression that the response process characteristized by the Rasch model is a sequential process. However, the Rasch model is NOT a sequential processing model but a static model, which just specifies the probability of a person with a given location responding, or being classified, in one of the categories of an item. 

For example, the term "step" is not used in the dichotomous case because it would imply, implausibly, that a person goes from being wrong to being right, or goes from disagreeing to agreeing. Instead the person is either wrong or right, or either disagrees or agrees; there is no sequential processing here. The response process is a classification into ordered categories defined by thresholds which can be seen as analogous to markings on a ruler except that the thresholds do not have to be equidistant as they are in a ruler - they are estimated. The threshold is the point where the probability of a response in either one of two adjacent categories is 50%. 

Disordered Thresholds as an Anomaly

As in the case of a ruler, thresholds marking off successive categories need to be ordered to be interpretable. However, in estimating the thresholds from the data, it is possible to discover that the estimates are not properly ordered. This is a sign that the categories are not working as intended and an anomaly in the data that needs to be understood and corrected is disclosed.

Prior to the work of Rasch, Thurstone had constructed a model for ordered categories which also involved thresholds. These may be derived from the Rasch thresholds. The problem with the Thurstone thresholds is that they are always ordered as a property of the model no matter what the features of the data - they have no use in disclosing whether categories are working in the ordering intended. Thurstone thresholds cannot disclose any anomalies in the ordering; indeed they will hide them. 

If you are interested to LEARN and KNOW more about Introductory Rasch Model Workshop: Applications in Survey Research and Educational Measurement, you can join our workshop.

The details of the workshops are as follow:

Course Title : Introductory Rasch Model Workshop: Applications in Survey Research and Educational Measurement

Date:  17 - 18 March 2014 (Monday - Tuesday)
Time:  8:30 am – 5:30 pm 
Medium:  English 
Fee:  RM300 (early bird rate) / RM400 (normal rate) 
Venue:  MPWS Training Centre, 63-1, 63-2, JalanKajangImpian 1/11,
Taman KajangImpian, Seksyen 7, 43650 Bandar BaruBangi, Selangor

For registration and more information, please go to the following link:

>> http://postgraduateworkshop.com/raschanalysis/

This workshop is open to all researchers, academicians and postgraduate students from government agencies, local and international higher learning institutions, as well as private sector.
 

We also accept payment via LO/PO. However, the price will be normal price and the LO/PO must be sent prior to the event. If your company/organization is having difficulty to prepare LO/PO prior to the event, we do accept Letter of Undertaking (LoU) as an assurance that the payment will be made after the event. 

Registration fees cover refreshments and lunch. All fees must be fully paid before the commencement of the course. Otherwise, participants will not be allowed to enter the lecture hall. 

Places are limited.

Only a few places.

Act now!  

For registration and more information, please go to the following link:

>> http://postgraduateworkshop.com/raschanalysis/