An IRT ana lysis of the personal optimism scale

In this study the psychometric properties of the Personal Optimism scale of the POSO-E questionnaire (Schweizer & Koch, 2001) for the assessment of dispositional optimism are evaluated by applying Samejima's (1969) graded response model, a parametric I item response theory (IRT) model for polytomous data. Model fit is extensively evaluated via fit checks on the lower-order margins of the contingency table of observed and expected responses and visual checks of fit plots comparing observed and expected category response functions. The model proves appropriate for the data; a small amount of misfit is interpreted in terms of previous research using other measures for optimism. Item parameters and information functions show that optimism can be measured accurately, especially at moderately low to middle levels of the latent trait scale, and particularly by the negatively worded items.