Page 241 - The Six Sigma Project Planner

Calculating Yields

The rolled throughput yield (RTY) summarizes defects-per-million-opportunities
(DPMO) data for a process or product. DPMO is the same as the parts-per-million
calculated by MINITAB. RTY is a measure of the overall process quality level or, as its
name suggests, throughput. For a process, throughput is a measure of what comes out
of a process as a function of what goes into it. For a product, throughput is a measure of
the quality of the entire product as a function of the quality of its various features.
Throughput combines the results of the capability analyses into a measure of overall
performance.

To compute the rolled throughput yield for an N-step process (or N-characteristic
TEAMFLY
product), use the following equation:
( 1,000,000) ( DPMO 2 ... 1,000,000)
DPMO
DPMO
n
1-
1
Rolled Throughput Yield = 1- x 1- 1,000,000) (
(Equation 1)

where DPMO is defects per million opportunities for step x in the process. For
x
example, consider a four-step process with the following DPMO levels at each step
(Table 24).

Table 24. Calculations Used to Find RTY

Process Step DPMO Defects per Unit (DPU) 1-DPU
=DPMO/1,000,000
1 5,000 0.005000 0.9950
2 15,000 0.015000 0.9850

3 1,000 0.001000 0.9990
4 50 0.000050 0.99995

Rolled Throughput Yield = 0.995 x 0.985 x 0.999 x 0.99995 = 0.979

Figure 48 shows the Excel spreadsheet and formula for this example. The interpretation
of the RTY is simple. If you started 1,000 units through this four-step process, you
would get only 979 units out the other end. Or, equivalently, to get 1,000 units out of
this process, you should start with 1,000 +=
1 1,022 units of input. Note that in a series
0.979
of processes or process steps the RTY is worse than the worst yield of any process or
step in the series. It is also worse than the average yield of 0.995. Many a process owner
is lulled into complacency by reports showing high average process yields. They are
confused by the fact that, despite high average yields, their ratio of end-of-the-line
output to starting input is abysmal. Calculating RTY may help open their eyes to what

Team-Fly
®
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