A paper by F. W. J. Olver [SIAM J. Numer. Anal. {\bf 15} (1978), no. 2, 368--393; MR0483379 (58 \#3385)] introduced new notations, and a new definition of relative error, into the analysis of error propagation. His work has made possible the application of a standard, rigorous approach to a wide class of numerical methods, in such a way that realistic error bounds are obtained. \par In the present paper the author describes Olver's approach clearly and concisely. He also introduces a novel notational device which provides a direct means of representing that error in an algebraic expression which is the result of an accumulation of rounding errors.