Six Sigma Metrics

For Six Sigma projects, the goal is to maintain statistical process control.  The measures most commonly used include:

• The Sigma level
• The Rolled Throughput Yield (RTY)
• Defects per Unit (DPU)
• Defects per Million Opportunities (DPMO)
• The First Pass Yield (FPY)

Consequential metrics are both business and process metrics. Consequential metrics measure anything that goes wrong as a result of improving the primary metric. The negative consequences are called consequential metrics.  Six Sigma concentrates on 4 types of process measurements.  Usually, all metrics and data held for a process can be summarized by all or some of these.  Measurements are made in terms of cost, quality, safety, or time.
The range of metrics within a process and factors that input into the function can therefore be defined by these 4 statements:

• Critical to Cost (CTC)
• Critical to Quality (CTQ)
• Critical to Schedule (CTS)
• Critical to Safety (CTS)    

The Critical to Cost metric refers to consideration of:

• The costs of inventory kept on hand
• The amount of labor put into production
• The cost of raw materials
• All sorts of overhead, such as employees and delivery costs

These costs are not just reflective of those borne by the business but even those that may be passed on to the customer in terms of charges or overall pricing policy.

CTQ refers to the accuracy or expected look of a product.  It may be one of the most familiar areas to individuals because it is often a good measure of customer satisfaction.  Example: A bakery is often viewed by the quality standards of its goods.

CTS relates to the time it takes to pass through one or more stages of a process.  It relates to delivery timing, time on hand of goods stocked, and arrival times for materials.

The purpose of the six sigma project is to meet the customers’ needs and be part of the overall business strategy. 
Many production environments measure their performance in terms of yield or their wastage rate.  If 100 products are created and 98 of them are usable, then the yield is calculated as 98%.  What is not accounted for in this metric is any reworking. What if every 100 unusable products are put through part of the process again, and again 98 of these become usable – it can make things difficult to understand.