In an attempt to simplify the application of work measurement data, the technique known as ‘time-slotting’, is used. Time-slotting consists of collecting data in a specified work area, dividing the whole time range into segments, or slots, and letting a single time, in each slot, represent the time standard of all of the jobs whose true standard times fall into the slot. This technique has been widely used in developing time standards for low repetitive tasks, such as in the area of maintenance, where research has been carried out on the behaviour of different methods of designing slotting scales. Nevertheless, there does not appear to be one generally accepted method of designing these time-slotting scales. In this paper the theory of time-slotting is considered in general terms. First the principal errors which may occur in time-slotting are delineated and then the ways in which two of these errors can be determined,a priori, are demonstrated. The effect of variables of the time-slotting scale upon each type of error is then discussed. The principles and equations evoked in the paper will provide the basis for the evaluation of time-slotting scales under many varying conditions. Furthermore, a means of separating component errors in computer simulation of time-slotting scales has been established.