Page 61 - Design for Six Sigma a Roadmap for Product Development

38 Chapter Two

section. For discrete data, we could directly calculate defective rate,
and then translate it to sigma level.

2.4.3. Stage 3: Analyze data and discover
causes of the problem
After data collection, we need to analyze the data and process in order to
find how to improve the process. There are two main tasks in this stage:

Data analysis. Using collected data to find patterns, trends, and
other differences that could suggest, support, or reject theories about
the cause and effect, the methods frequently used include
■ Root cause analysis
■ Cause–effect diagram
■ Failure modes–effects analysis (FMEA)
■ Pareto chart
■ Validate root cause
■ Design of experiment
■ Shanin method
Process analysis. This involves a detailed look at existing key
processes that supply customer requirements in order to identify
cycle time, rework, downtime, and other steps that don’t add value
for the customer. We can use process mapping, value stream map-
ping, and process management methods here.

2.4.4 Stage 4: Improve the process
We should have identified the root causes for the process performance
problem after completing stage 3.
If the root causes of process performance problems are identified by
process analysis, the solutions are often featured by techniques such as
process simplification, parallel processing, and bottleneck elimination. If
the root causes are identified by applying data analysis, then sometimes
finding the solution to performance problem is easy. There are some cir-
cumstances that finding the solution is very difficult, because many
“obvious” solutions may potentially solve the problem, but will have
harmful effects on other aspects of the process. In this case, creative solu-
tions need to be found. Brainstorming and TRIZ may be used here.

2.4.5 Stage 5: Control the process
The purpose of this stage is to hold on to the improvement achieved
from the last stage. We need to document the change made in the
improvement stage. If the improvement is made by process manage-
ment methods, such as process simplification, we need to establish a