A Type System Based on End-User Vocabulary

In previous work we have developed a system that automatically checks for unit errors in spreadsheets. In this paper we describe our experiences using the system in a workshop on spreadsheet safety aimed at high school teachers and students. We present the results from a think-aloud study we conducted with five high school teachers and one high school student as the subjects. The study is the first ever to investigate the usability of a type system in spreadsheets. We discovered that end users can effectively use the system to debug a variety of errors in their spreadsheets. This result is encouraging given that type systems are difficult even for programmers. Since the subjects had difficulty debugging "non-local" unit errors, we devised new methods to improve the error-location inference. We also extended the system to generate change suggestions for cells with unit errors, which when applied, would correct unit errors. These extensions solved the problem that the study revealed in the original system.