Croatian 1st-year and 3rd-year high-school students (N = 170) completed a conceptual physics test. Students were evaluated with regard to two physics topics: Newtonian dynamics and simple DC circuits. Students answered test items and also indicated their confidence in each answer. Rasch analysis facilitated the calculation of three linear measures: (a) an item-difficulty measure based upon all responses, (b) an item-confidence measure based upon correct student answers, and (c) an item-confidence measure based upon incorrect student answers. Comparisons were made with regard to item difficulty and item confidence. The results suggest that Newtonian dynamics is a topic with stronger students' alternative conceptions than the topic of DC circuits, which is characterized by much lower students' confidence on both correct and incorrect answers. A systematic and significant difference between mean student confidence on Newtonian dynamics and DC circuits items was found in both student groups. Findings suggest some steps for physics instruction in Croatia as well as areas of further research for those in science education interested in additional techniques of exploring alternative conceptions.