Measurements in quantum mechanics are often taught in an abstract, theoretical context. Compared to what is known about student understanding of experimental data in classical mechanics, it is unclear how students think about measurement and uncertainty in the context of experimental data from quantum mechanical systems. In this paper, we tested how students interpret the variability in data from hypothetical experiments in classical and quantum mechanics. We conducted semi-structured interviews with 20 students who had taken quantum mechanics courses and analyzed to which sources they attribute variability in the data. We found that in the quantum mechanics context, most students interpret any variability in the data as irreducible and inherent to the theory. While acknowledging the influence of experimenter error, limited resolution of measurement equipment, and confounding variables (like air resistance) in classical mechanics, many students did not recognize the influence of such effects in quantum mechanics. Some students expressed the view that there are inherently fewer confounding variables in Quantum Mechanics and the equipment used is more precise. We derive tentative implications for instruction and propose further research to test the influence of framing on the responses to our interview protocol. © 2019, American Association of Physics Teachers. All rights reserved.