Using action theory, the error detection of computer users was investi
gated. For reasons of internal and external validity both a field stud
y and two laboratory experiments using word processing were carried ou
t. Research questions pertained to the error detection rate of various
types of errors, the impact of task complexity on error detection, an
d the question of computerised error detection. The results showed a h
igher error detection rate (96.5%) in the field study than in the expe
riments. Errors occurring in complex tasks were more difficult to dete
ct. Moreover, 50% of all errors could be detected only by the users. N
ot more than 22% of the errors were detected with the help of the comp
uter system. Errors in complex planning were the most difficult for co
mputer supported error detection. As a practical consequence, a strate
gy of error management is suggested which calls for the support of the
process of error recovery through software design.