The application of Item Response Theory (IRT in advance) has been increasing on psychometric research. Nevertheless the use with the measurement of sociological and psychological variables is not that common and the major limitations of IRT is in the special case of the measurement instrument with polytomous items (Asún & Zuñiga, 2008), this is our case. These are measurement instruments related with variables as attitudes, motivations, interest, etc. where the difference between response categories is the intensity of agreement with the feature presented in the item.

The main objective of the present research is show the advantages and limitations of the use of Rash polytomous models, specifically the Partial credit and the Rating scale models, in the validation of Perception Scale of Child Well-being. In one hand the IRT provides advantages over Classical Test Theory like the invariance of the parameters of items in relation with the sample or the test independent estimation of latent scores. In the other hand obtaining indicators to analyze the living conditions of children is a growing concern, select the best methodological choice in order to validate a Perception Scale of Child Well-being is an important educational research goal, transferable to another areas of social, political and psychological concerns.

In order to obtain this target, we compare forth polytomous IRT model. We use the simple logistic model of Rasch for the model estimation, the differences between these four models is twofold:

1. The procedure to estimate the probabilities of individuals answers in the different item categories: Partial Credit Model (PCM) (Masters, 1982) / Rating Scale Model (RSM) (Andrich, 1978).
2. Dimensionality of the latent structure of measurement instrument: Multidimensional five category model/ Unidimensional model.

The combination of these two features leads to the four models analyzed in the present research: a) PCM- Unidimensional; b) PCM-Multidimensional; c) RSM-Unidimensional; d) RSM-Multidimensional.

This study seeks the contrast between these models through the validation of the PSCWI (Perception Scale of Child well-being Indicators)instrument. Different national and international agencies and institutions have directed considerable efforts to obtain accurate measurements of the specific characteristics of child well-being. At the same time, the views of future education professionals about the determinants of child well-being and its impact on the lives of children help closer to this reality and understanding it more deeply. The instrument PSCWI has been made to know what are the determinants of child well-being recognized by education professionals trainees, as well as the importance degree given to each of them. Pick out the best option to validate the EPIBI instrument is essential in order to obtain a good certainty of the functioning of this.

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