Functionality or user interface: Which is easier to learn when changed?

The rapid release model of software introduces frequent updates to the existing software every twelve-eighteen weeks, forcing a user to get accustomed to its new features. We propose an experimental study to compare the learning gaps that are introduced when a user is exposed to a new version of a software, with which he is previously acquainted. In order to explore the problem, we propose four models of a machine, with each model involving an update either to the functionality, or to the user interface, or both. We conducted a between-subjects experimental study with thirty-two participants who performed two tasks successively on two models of a machine, the second one being a updated model of the first. The analysis of the data using ANOVA implies that a change in the user interface dominates a change in the functionality. Results indicate that 88% of the errors were caused due to a change in the user interface. 87.5% of the users who underwent a change in the user interface hold this change responsible for the learning gap, while only 56.25% users who underwent a change in the functionality consider it to be a potential reason for the learning gap.