This paper provides estimates for the average rate of occurrence of fa
ilures, ROCOF (''failure rate''), for critical failures when also degr
aded failures are present. The estimation approach is exemplified with
a data set from the offshore equipment reliability database ''OREDA''
. The suggested modelling provides a means of predicting how maintenan
ce tasks will affect the rate of critical failures. (C) 1998 Elsevier
Science Limited.