The level of measurement of trust in automation

Psychometrics are increasingly used to evaluate trust in the automation of systems, many of them safety-critical. There is no consensus on what the highest level of measurement is for trust. This is important as the level of measurement determines what mathematics and statistics can be meaningfully applied to ratings. In this work, we introduce a new method for determining what the maximum level of measurement is for psychometrically assessed phenomenon. We use this to determine the level of measurement of trust in automation using human ratings about the behaviour of unmanned aerial systems performing search tasks. Results show that trust is best represented at an ordinal level and that it can be treated as interval in most situations. It is unlikely that trust in automation can be considered ratio. We discuss these results, their implications, and future research.