How do you handle uncertainty or variability in the inputs or outputs of the process flow diagram?

1 Define the scope

The first step of conducting a PFMEA is to define the scope of the process and its boundaries. You need to specify the input and output variables, the customer requirements, the process steps, and the process functions. You can use a PFD to illustrate the flow of materials, energy, information, and resources through the process. A PFD can also help you identify the sources and types of uncertainty or variability in the inputs or outputs, such as variations in quality, quantity, timing, temperature, pressure, or composition.

3 Evaluate the effects and severity

The third step is to evaluate the effects of each failure mode on the process and the customer. You need to consider how the failure mode can impact the quality, safety, cost, or delivery of the product or service. You also need to assess the severity of the effect, which is a measure of how serious or harmful the effect is. You can use a scale from 1 to 10, where 1 is no effect and 10 is catastrophic effect, to rate the severity of each failure mode.

5 Implement the controls and detection

The fifth step is to implement the controls and detection methods for each failure mode and its causes. A control is a preventive or corrective action that reduces or eliminates the risk of the failure mode or its cause. For example, a control for a missing input variable could be a verification check, a backup supply, or a contingency plan. A detection method is a way of monitoring or testing the process or the product to identify the failure mode or its cause before it affects the customer. For example, a detection method for a defective output variable could be a visual inspection, a sampling procedure, or a quality audit. You need to describe the existing or planned controls and detection methods for each failure mode and its causes. You also need to evaluate the detection of each cause, which is a measure of how easy or difficult it is to detect the cause before it leads to the failure mode. You can use a scale from 1 to 10, where 1 is very easy and 10 is very difficult, to rate the detection of each cause.

6 Calculate the risk priority number

The final step is to calculate the risk priority number (RPN) for each failure mode and its causes. The RPN is a numerical value that indicates the relative priority of addressing the failure mode or its cause. The RPN is calculated by multiplying the severity, occurrence, and detection ratings. The higher the RPN, the higher the risk. You can use the RPN to rank the failure modes and their causes and to prioritize the actions for improvement. You can also use the RPN to monitor the progress and effectiveness of the actions and to update the PFMEA as needed.