A number of activities in a distributed system generate files describing the details of each activity, known as log files. For example, the process of logging in to a computer generates entries in a log file which monitor activities that could threaten a system security. The process of analysing a log file involves producing summary listings and any exception reports about unusual activities: for example, a user logging in at the dead of night. There are other uses of log file analysis over and above those associated with system and network administration. For example, a Web server generates information about the hits that are made on Web pages in a Web log file; this is commercially useful in that it provides data on who is visiting a Web site and what pages are popular and enables the Webmaster to structure the Web pages so as to maximize the number of visitors.