“Failure limited” data and TTT-based trend tests in multiple repairable systems

Tests for trend in multiple repairable systems based on the total time on test (introduced by Kvaløy and Lindqvist) are valid for data that end at a failure time only if data collection was planned to continue until a predetermined number of failures had occurred. They are not valid for a commoner form of data that end with the last of a random number of failures within a predetermined observation period. The necessary modification to the analysis is given here and appropriate terminology is suggested.