The Force Concept Inventory (FCI) is an important diagnostic instrument widely used in the field of physics education research. It is therefore important to evaluate and monitor its functioning using different statistical analysis tools. One such tool is the stochastic Rasch model, which enables construction of linear measures for persons and items from raw test scores and which can provide insight into the structure and functioning of the test (how item difficulties are distributed within the test, how well the items fit the model, and how well the items work together to define the underlying construct). The data for this analysis comes from research conducted in 2006-07 on 1676 Croatian high school students (age 17–18 years). The students' results on the FCI were analyzed with the Rasch measurement computer software WINSTEPS 3.66. The average FCI score for the sample was found to be (27.7±0.4)%, indicating most students were still non-Newtonians at the end of high school despite the fact that physics is a compulsory subject in Croatian schools. Since the FCI is routinely used as pretest and post-test on two different population types (non-Newtonian and predominantly Newtonian), an additional predominantly Newtonian sample (N=141, average FCI score of 64.5%) of first year students enrolled in an introductory physics course at the University of Zagreb was also analyzed. The Rasch model based analysis suggests the FCI has succeeded in defining a sufficiently unidimensional construct for each population. The analysis of fit of data to the model found no grossly misfitting items which would degrade measurement. Some items with larger misfit and items with significantly different difficulties in the two samples of students require further examination. The analysis revealed some problems with item distribution in the FCI and suggested the FCI may function differently between non-Newtonian and Newtonian populations. Some possible improvements of the test are suggested.