There are many Six Sigma metrics and/or measures of performance used by Six Sigma practitioners. In addition to the ones we’ll cover here, several others (Sigma level, Cp, Cpk, Pp, Ppk, takt time, cycle time, utilization etc.) will be covered in other articles. The Six Sigma metrics we will cover in the next two articles will be:
- Defects per Unit (DPU)
- Defects per Million Opportunities (DPMO)
- Yield (Y)
- Rolled Throughput Yield (RTY)
Before we dive into the metrics themselves we must touch on the concept of “metrics”. All projects or programs have purposes and those purposes must be measureable to be effectively managed. Six Sigma uses the transfer function as an approach to achieving improvements. That approach mandates the need for a measureable primary metric. So what is a primary metric?
The primary metric is a generic term for a Six Sigma project’s most important measure of success. The Primary metric is defined by the Black Belt, GB, MBB or Champion. A primary metric is an absolute MUST! For any project and it should not be taken lightly. Here are a few characteristics of good primary metrics:
- Primary metrics should be tied to the problem statement
- Primary metrics should be measureable
- Primary metrics should be expressed with an equation
- Primary metrics should be aligned to business objectives
- Primary metrics should tracked at the proper frequency (hourly, daily, weekly, monthly etc.)
- Primary metrics should be expressed pictorially over time with a run chart, time series or control chart
- Primary metrics should be validated with an MSA
The primary metric is the reason for your work, it’s the success indicator and beacon for your project. The primary metric is of utmost importance and is the focus of your project BUT…not at the expense of your secondary metric.
The secondary metric is the thing you don’t want sacrificed on behalf of a primary improvement. A secondary metric is one that makes sure problems are not just “changing forms” or “moving around”. The secondary metric keeps us honest and ensures we’re not sacrificing too much for our primary metric. If your Primary Metric is a cost or speed metric then your Secondary Metric should probably be some quality measure.
- Example: If you were accountable for saving energy in an office building and your primary metric was energy consumption then you could shut off all the lights and the HVAC system and save tons of energy…..except that your secondary metric(s) are probably comfort and functionality of the work environment.
Primary and Secondary Metrics are simply guideposts for your Six Sigma project. They can be one of many measures that are important to the business (sales, throughput, time, speed, weight, length, velocity, accuracy etc. Let’s now turn our attention to a few common Six Sigma type metrics starting with Defects per Unit.
Defects per Unit: DPU
DPU stands for “Defects per Unit“. DPU is the basis for calculating DPMO (Defects per Million Opportunities) and RTY (Rolled Throughput Yield) which we’ll cover below. DPU is found by dividing total defects by total units:
- DPU = D/U
For example if you have a process step that produces an average of 65 defects for every 598 units then your DPU = 65/598 = 0.109 (10.9%). Another way of referring to DPU would be calling it a ‘defect rate’. What might your defect rate be if you counted 34 defects out of 212 units sampled? if you answered 16% you would be correct.