This is an excel file containing the numerical data resulting from the scoring of the videos recorded during each experimental session. The videos are not available for ethical reasons. The excel file includes several spreadsheets, whose naming corresponds to the various set of results reported in the publication (Open-ended ‘mental set’ tasks: An alternative approach to studying design fixation). In each spreadsheet, the column headings are self-explanatory, provided you also have the corresponding publication given above as a reference. The data was collected from July to October 2016 from 40 undergraduate and postgraduate students with an engineering background at the University of Cambridge, UK. Participants were tested individually and received £10 for your participation. They were initially told that the study aimed at investigating how people played computer games. The real aim of the study was revealed to the participants at the end of each experimental session, during the debriefing phase. Demographic data was collected from the participants and are reported in the first spreadsheet of the file. The study was approved by the local Ethical Committee and the participants signed a Consent Form before starting with the experiment.