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Frequently designers of computer systems have few (if any) convenient means of investigating system per- formance during actual operation. This kind of analy- sis must, of course, take into account user demands and characteristics of the host system's hardware/software complement. Moreover such studies are complicated because they typically involve a large number of variables which, because of their random nature, do not exhibit unique values. The problem is not only to identify criteria which characterize the manner in which the system is used but also to select parameters which are readily measurable. This presentation describes the approaches employed in characterizing and instrumenting the usage and servicing capacity of two operating systems designed for use on Xerox Sigma computer systems. These operating systems are the Batch Time-sharing Monitor (BTM) and the Universal Time-sharing System (UTS). The performance monitors are designed with emphasis on sampling, sorting and ordering of statistical data. Examples of performance monitoring data are presented which were obtained from actual measurements.