The assessment of probabilistic reasoning skills is important in various context. The aim of this study was to develop an abbreviated, open-ended version of the Probabilistic Reasoning Scale (PRS-B), applying Item Response Theory (IRT). The analyses based on the open-ended version of the 16-item scale suggested the exclusion of seven items that did not perform well in measuring the latent trait. The resulting 9-item scale (PRS-B), which included highly discriminative items, covered a wider range of the measured trait than the original scale and it showed high measurement precision. Concerning validity, the results showed the expected correlations with numerical skills, math anxiety, and statistics achievement. In conclusion, the PRS-B can be is a shorter, open-ended version of the PRS that maintains excellent reliability and validity.