We present a tutorial on a power spectral approach to variability in serial motor performance, describing as a case study two experiments on the form of the variance in two force production tasks. In Experiment 1 we examine grip force and load force in repetitive unimanual pulling; in Experiment 2, we describe repetitive bimanual pressing. In both experiments log-log plots of power spectral density of peak force of the responses in each stream against frequency (i.e. periodicity or repetition cycle defined with respect to the ordered succession of responses) were approximately linear with negative slopes which varied systematically with test conditions. We propose the two response streams in each experiment are associated with different levels of sustained attention and state a simple model involving summation of moving average processes on multiple time scales as a qualitative account of the changes in 1/f slope.