A method of handling performance data comprising a set of events is described. An event record for each event is stored as a set of blocks, each block containing one or more attributes of the event. The storage space occupied by each event record in is then reduced in discrete steps, each step including a reduction process that reduces the size of one of the set of blocks. This enables the provision of intermediate records between events and counters so that new event records contain complete details of their event, older event records contain less information, and even older event records may contain only high-level (counter) information.