We have looked that it’s essential to define a project chart to determine the project’s target clearly. To do that, it’s necessary to define the correct metrics to assess the problem. In this chapter, we will look at the most common.
Defect per unit
It measure the defect that you find in relation of how many unit (element) you inspect. So the formula is simple:
Number of defect / Unit inspected
Example: if you inspect 1000 donuts and find ten without holes, you have 10/1000 = 0,01 defect per unit.
Defects per Million Opportunities
Defects per million opportuinities is based onf the Defect per opportunity that is:
DPO = Defect / opportunities
- Defect: is the number of defects that you found, one object inspected can have more than one kind of defect and each one is counted singularly ;
- Opportunities: is the different kind of defect that you can find multiplied for the number of element inspected;
DPO give you a percentage. for the defect per million opportunities you need to do that:
DPMO = DPO * 1 million
First Time Yield
The number of defect-free units produced compared to all the attempts to make. So the formula is:
FTY = Defect-free unit / number of unit entering the process
You can calculate it for all the processes simply calculating the FTYi of every step of the process and then multiplying them. In this case the formula become:
FTY of Process = FTY1 * FTY2 * .. * FTYn (where n is the number of step of the process)
Rolled Throughput Yield
Help to asses the health of a process giving you the probability that an element is product well at the first time.
RTY = number of unit in input – (scrap + reworks) / number of unit in input
So it differ from the FTY because you remove all the unit that you don’t do at the first attempt.
It is simply the time needed to do one unit from the start to the end. So it’s the time between the finish of one unit and the other. If you have a multiple-step process, the cycle time is determined by the step requiring more time, also called a bottleneck.
Is the percentage of time that the process works.
Change Over Time
Refers to the ability to quickly start the next work item as soon as the current one is finished.