Lusser's law in systems engineering is a prediction of reliability. Named after engineer Robert Lusser, and also known as Lusser's product law or the probability product law of series components, it states that the reliability of a series of components is equal to the product of the individual reliabilities of the components, if their failure modes are known to be statistically independent. For a series of N components, this is expressed as:
:<math>R_s=\prod_{i=1}^N r_i=r_1 \times r_2 \times r_3 \times ... \times r_n</math>
where R<sub>s</sub> is the overall reliability of the system, and r<sub>n</sub> is the reliability of the n<sup>th</sup> component.
If the failure probabilities of all components are equal, then as Lusser's colleague Erich Pieruschka observed, this can be expressed simply as:
For example, given a series system of two components with different reliabilities — one of 0.95 and the other of 0.8 — Lusser's law will predict a reliability of
:<math>R_s = 0.95 \times 0.8 = 0.76</math>
which is lower than either of the individual components.