P-diagrams are used to design robust systems, which are either products or process functions that are insensitive to the presence of noise factors. P-diagrams were first used in electronics design to understand signal-to-noise ratios. They provide a faster, more logical way to identify potential failure modes than moving down through functional FMEA items line by line.

Using a boundary diagram, you define the system and interactions that p-diagrams are to analyze. Typically, you create a p-diagram for each system function. By using p-diagrams to evaluate system interfaces and noise factors prior to building the FMEA, you are much less likely to overlook potential failure mode causes.

A p-diagram shows the inputs or signal factors that must pass through the system to produce ideal functional outputs. These outputs are the responses that the customer expects the system to deliver. Because noise factors prevent the system from achieving ideal functional outputs, design engineers determine how to apply control methods to reduce the impact of noise factors. Error states are the undesirable outputs that occur due to noise factors that cannot be eliminated.

The objective of robust system design is for the actual responses to be as near the ideal functional outputs as possible. For more information, see: