The purposeful processing of long-term documented maintenance data can provide plenty of information not only about a machine's history, but also about its maintenance system. The main objective of data analysis is to continually improve the maintenance efficiency, which is closely related to improvements in dependability and overall productivity of the production equipment. This paper pinpoints the main problems of most widespread indicators of maintenance efficiency - OEE and other proposed alternative methods for maintenance efficiency assessment, with the aim of locating the weakest areas of the maintenance system.