Error analysis of ticket vending machines: comparing analytic and empirical data

An error analysis was performed on the three ticket vending machines installed at London underground and overground train stations. A brief analytic inspection, resulting in a set of predicted errors, was followed by lengthy empirical observations of successes, failures and errors occurring during machine use. There were two observational phases, 5 years apart. Comparisons were made between the patterns of error-making on the three machines, using error categories derived from the initial analysis. It was found that these comparisons were sufficient to account for most of the between-machine and between-phase differences, although some unattributed errors remained. It was also found that much of the observed pattern of error-making had been predicted by the initial inspection, and it is suggested that, for relatively simple interfaces such as these, the method (Dialogue Error Analysis) is sufficient to identify and prioritize most problems that will occur in use. Attempt was also made to relate the observed error categories to influential taxonomies of human error.