8. Assessing Product Reliability
8.1. Introduction

## How can you evaluate reliability from the "bottom-up" (component failure mode to system failure rate)?

Several simple models can be used to calculate system failure rates, starting with failure rates for failure modes within individual system components This section deals with models and methods that apply to non-repairable components and systems. Models for failure rates (and not repair rates) are described. The next section covers models for (repairable) system reliability growth.

We use the Competing Risk Model to go from component failure modes to component failure rates. Next we use the Series Model to go from components to assemblies and systems. These models assume independence and "first failure mode to reach failure causes both the component and the system to fail".

If some components are "in parallel", so that the system can survive one (or possibly more) component failures, we have the parallel or redundant model. If an assembly has $$n$$ identical components, at least $$r$$ of which must be working for the system to work, we have what is known as the r out of n model.

The standby model uses redundancy like the parallel model, except that the redundant unit is in an off-state (not exercised) until called upon to replace a failed unit.

This section describes these various models. The last subsection shows how complex systems can be evaluated using the various models as building blocks.