Any aspirant preparing for Lean Six Sigma Certification Exams of ASQ or IASSC, it is important for you to understand the difference between CTQ, Primary Metric and Secondary Metric.
In Lean Six Sigma, the term CTQ is very commonly used. CTQ is an acronym Critical to Quality. As the name suggests, any attribute, parameter, factor or metric that is critical to the quality of the product or service you are offering to your customers can be considered as CTQ.
One important element of CTQ is that it should be measurable, directly or indirectly. As we all know, anything that cannot be measured, cannot be achieved. More or less, it is difficult to maintain the desired level of quality for any product or service, if an important or critical attribute is not measured.
In the context of improvement projects in Lean Six Sigma, using the framework of DMAIC, the objective of such projects will be for Lean Six Sigma Green Belt or Lean SIx Sigma Black Belt will be to improve a given CTQ. For example, if on time delivery could be a CTQ for the business because the customer wants your products or service on-time. Thus CTQ emanates from the customer. There are usually many CTQs for any product or service. But the DMAIC Project will have to focus on improving only CTQ at a time. Especially the one which is not performing up to the mark, as desired by customers. This becomes the primary metric of the project. In this example, On-time delivery could be a Primary Metric. The purpose of the project will now be fully aligned to improve this primary metric.
If that is the primary metric, the question we need to answer is what is a secondary metric in a Lean Six Sigma Improvement project? To understand how to identify secondary metric, we will take a step back and talk about the purpose of improvements in using Lean Six Sigma or any other structured problem solving approach.
There are many benefits of using such a framework, but come to the most pertinent point. Any improvement that is brought about by compromising or reducing the performance of any aspect of the business, product or service is not real improvement. It is mere optimization and doesn’t need problem solving skills. For example, if one adds more employees, equipment or cost to reduce the on-time delivery, that isn’t true improvement.
Hence we will need to ensure that Cost of Servicing and Quality of Service does not deteriorate as we try to improve On-time delivery.
These become Secondary Metrics for the project. Thus Secondary Metrics are those parameters or CTQs that will be monitored and sustained at current level, as we try to improve the primary metric. Secondary Metrics are also called Consequential Measures or Counter-measures.
While any DMAIC project should ideally have only one Primary Metric, it can have as many secondary metrics. All secondary metrics will be monitored and throughout the project, care will be taken to ensure there are no side effects on the Secondary Metrics as a result of any improvement in Primary metric.
The best way to identify secondary metrics will be through brainstorming. An alternate way would be to look for statistical relationships between various CTQs.