Managing Process Defects

Working on process improvement projects requires understanding what a defect means and how to manage it. A defect is said to have occurred when the outcome of a process is not as expected. The term “expected outcome” here, refers to requirements we have specified in advance. An error or discrepancy in a process can lead to the creation of defective parts.   

Defects should clearly be avoided or minimized as they lead to wasted resources in the form of scrap, labour, energy consumption, machine capacity and materials. These are referred to as internal failure costs.

If a defective product goes out to the customer, a business can have external failure costs in the form of warranty, recall and loss of reputation which may lead to a need to reduce prices and incure losses.

Important Formulae For Managing Defects

An item is only defective when it is considered unacceptable due to one or more defects.

Fraction defective/defect rate/failure rate is the number of defective units at a process step divided by the total number of produced units going into the process step. Yield is the number of good units produced at a process step divided by the total number of units going into the process step and can be calculated using the formula, 1 - Defect Rate

If 

X = Number of process steps or parts

Yield at each process step = 1 - defect rate 

Overall Process Yield = (1- defect rate) raised to the power of X, 

For different yields at each process step, you multiply the yields for each step together. For example, overall process yield = (yield for step a) (yield for step b) (yield for step c)

The Parts Per million (PPM) metric can also be used for measuring the number of defects in a process and can be calculated using the formula below:

PPM = No. of defective units * million units 

DPU = Defects per unit = average number of defects per unit of product, that is Total no. of defects/Total no. of units

DPMO = Defects per million opportunities = ((DPU * 1000000)/Total opportunities for a defect in one unit)

Managing Defects

Defects should be proactively managed to ensure they do not end up costing the business significant resources to fix. Eliminate defects in your organization by creating more efficient business processes through standardization and continuous improvement. Here are some quick tips for managing defects:

• Detect early – If defects are detected early, the business is better able to reduce them and implement corrective measures which can save resources down the track

• Identify the root causes of defects so that proposed solutions are long-lasting

• Foster an organizational culture of bringing attention early to problems

• Create in-built tests that can proactively spot defects as part of business processes, where possible.