Combined random and swept sinusoidal vibration is used in the qualification of missile electronic equipment. The subject of this report is how structural fatigue produced by a combined excitation is related to structural fatigue produced by another combined excitation or to that produced by individually applied random or sinusoidal excitation. In particular, a method is given by which different random or combined random and sinusoidal vibration excitations can be compared. This method finds practical application in comparing vibration specifications and tests, and for the purpose of modifying vibration requirements from one type to another.In this derivation, substitute sinusoidal vibration excitations and sweep rates are based on the response characteristics of a single resonator system, while amplitude and time relationships are based on the S‐N diagram of the material under consideration. Equations and curves are given and their use illustrated by means of a numerical example.