| Six
Sigma Metrics Overview |
|
Six Sigma uses a number of specialized
measures (metrics). The most fundamental
is the Defects Per Million Opportunities
(DOMO) measure. The aim of six sigma is
to achieve less than 3.4 DPMO. A process
that gives 3.4 DPMO has achieved 'six sigma'
The Rolled Throughput Yield (RTY) is a
key tool in achieving this. The individual
yields at each process step might look,
at first site, quite good, but the First
Pass Yield might be significantly lower.
Even when the First Pass Yield at each process
step looks satisfactory the overall or Rolled
throughput Yield may be low.
The sigma metric is an alternative to the
traditional process capability and process
performance measures used in Statistical
Process Control.
| Defects
per Million Opportunities |
|
The Six Sigma approach aims to get the DPMO
below 3.4 per million. Using the number of
opportunities for a defect, rather than the
number of units with a defect, allows the
measure to be applied to simple and complex
items. The disadvantage is that the definition
of ‘opportunity’ is hard to pin down. For
example it could be (incorrectly, but plausibly)
argued that recording an address involves
many opportunities for a defect; wrong street
number, wrong zip, street name spelled wrong,
and so on. The opportunity should be defined
in terms the customer cares about.
The total number of defects observed when
processing a number of units. Suppose you
observe 100 units being made. Five fail,
four are reworked, and one is scrapped.
The DPU is 0.05.
By conventional thinking the process yield
is 99%. However the First Pass Yield (the
proportion of units that go through the
process the first time) is:
See Defects Per Million Opportunities
See Defects Per Unit
The proportion of units that, on average,
go through a process first time without
defects. It is calculated from:
See also Defects per Unit
See First Pass Yield
The probability that a unit can pass through
a process without defects. It is the product
of the first pass yields at each step:
RTY = y1 x y2
x y3 x ………..yn
where the yi values are the
yields at each step before rework.
The argument is that the perceived yield
is misleading because it ignores the rework.
In process rework is sometimes known as
the 'hidden factory'. Consider an example:
| |
Defects |
Scrap |
Rework |
Units |
DPU |
FPY |
RTY |
| 1 |
10 |
5 |
5 |
100 |
0.100 |
0.905 |
0.905 |
| 2 |
12 |
5 |
7 |
95 |
0.126 |
0.881 |
0.797 |
| 3 |
8 |
0 |
8 |
90 |
0.089 |
0.915 |
0.730 |
| 4 |
10 |
5 |
5 |
90 |
0.111 |
0.895 |
0.653 |
| 5 |
5 |
1 |
4 |
85 |
0.059 |
0.943 |
0.616 |
| Totals |
45 |
|
|
460 |
0.485 |
|
0.616 |
See Rolled Throughput Yield
The standard deviation is usually represented
by the Greek letter sigma (s).
See Sigma Level
The Six Sigma methodology measures the
capability of a process using the 'sigma
level'. The aim is to achieve a sigma level
of at least six, which equates to less than
3.4 DPMO.
The Sigma level originates
from the normal distribution. The idea is
that the span between the upper and lower
specification limits should be at least
12 standard deviations (six standard deviations
on each side). Because it is not usually
practical to set the processes mean exactly
on target, and the mean of most processes
is subject to drift, a 1.5 standard deviation
offset is assumed in the calculation.
Other sigma levels are:
| Sigma
Level |
Parts
Per Million (PPM) |
| 3 |
66811 |
| 4 |
6210 |
| 5 |
233 |
| 6 |
3.4 |
The proportion of a process output that
is free of defects.
|
