In summary, the use of external workers with a response time decreases the efficiency under CBM and more so in the serial configuration than in the parallel configuration. By contrast, under ABR it also increases the grouping of maintenance activities, which absorbs the negative effect of longer response times on efficiency in the serial configuration. Furthermore, since more PM activities are scheduled under ABR than under CBM, the chance that a CM activity can be combined with a scheduled PM activity during the response time is larger. This limits the average delay time under ABR, which positively affects the efficiency.