Page 225 - Six Sigma Demystified

Part 3 s i x s i g m a to o l s 205

Note that some statistical process control (SPC) software will allow varying
sample sizes for the C chart. In this case, the control limits and the average line
will be adjusted for each sample. Many times it is less confusing to use a U chart
for these data because only its control limits will vary (the average line will
remain constant). See “Statistical Process Control (SPC) Charts” and “Run-Test
Rules” for more detail.

Cause-and-effect Diagram

Cause-and-effect diagrams are graphic brainstorming tools. Listing all the causes
for a given effect in a clear, organized way makes it easier to separate out po-
tential problems and target areas for improvement. These charts sometimes are
referred to as fishbone diagrams because of their form: Causes are listed on lines
that branch off from the effect in much the same way a fish’s ribs branch from
its spine. They are sometimes called Ishakawa diagrams in reference to a Japa-
nese engineer who popularized their use for quality improvement.

When to Use

Analyze Stage
• To brainstorm potential underlying process factors that can be investi-
gated in a designed experiment

Improve Stage

• To generate a list of potential failure modes that should be addressed in
the solution

Methodology
Begin by brainstorming the potential relationships between the process and the
outcome. The outcome, or effect, typically is stated in terms of a problem rather
than a desired condition, which tends to help the brainstorming.
The major branches of the fishbone are chosen to assist in brainstorming or
to categorize the potential problems afterwards. You may find it convenient to
use the 5M’s and E (manpower, machines, methods, material, measurement,
and environment) or the 4P’s (policy, procedures, plant, and people) to either
categorize on the final fishbone or ensure that all areas are considered during
brainstorming. Categorizing the potential causes (as branches off the spine) can