  Modern high-energy physics experiments collect data using dedicated complex
multi-level trigger systems which perform an online selection of potentially
interesting events. In general, this selection suffers from inefficiencies. A
further loss of statistics occurs when the rate of accepted events is
artificially scaled down in order to meet bandwidth constraints. An offline
analysis of the recorded data must correct for the resulting losses in order to
determine the original statistics of the analysed data sample. This is
particularly challenging when data samples recorded by several triggers are
combined. In this paper we present methods for the calculation of the offline
corrections and study their statistical performance. Implications on building
and operating trigger systems are discussed.
