This paper investigates the impact of different levels of expertise on errors in human-computer interaction. In a field study 174 clerical workers from 12 different companies were observed during their normal office work and were questioned on their expertise with computers. The level of expertise was determined by (a) the length of time an employee had worked with a computer (computer expertise); (b) the number of programs she knew (program expertise); and (c) the daily time s/he spent working with the computer (daily work-time expertise). These different operationalizations of novices and experts led to different results. In contrast to widespread assumptions, experts did not make fewer errors than novices (except in knowledge errors). On the other hand, experts spent less time handling the errors than novices. A cluster analysis produced four groups in the workforce: occasional users, frequent users, beginners, and general users. © 1992 Taylor & Francis Group, LLC.