Quite often the product of design is assessed in interaction design education, but we need to develop criteria also for courses that focus on learning to conduct and manage the design process. An earlier approach to set grading criteria has been grounded in the SOLO (Structure of the Observed Learning Outcome) taxonomy. Students need, however, to learn practitioners' criteria, rather than teachers' criteria, to make a successful transfer to practice. One way of achieving that is to align criteria with the conceptions of design process quality used by professional interaction designers. The question is then what those conceptions are, and how they can be accounted for in assessment criteria for projects in interaction design education. A phenomenographic research method was used, and interviews were conducted with ten experienced interaction designers. The interviews were analyzed using qualitative content analysis. The results show that professional interaction designers see design process quality as inspiration, a well-grounded rationale, employment of established methods, and constraints management. These conceptions are mapped to a criteria-referenced grading scale. The criteria should, with careful transfer, be applicable also in other design disciplines.