Page 308 - Six Sigma Demystified

288 Six SigMa DemystifieD

the null hypothesis. When software is used, the calculated p value provides a direct
estimate of the probability of obtaining results as extreme as (or more extreme
than) the sample data if the null hypothesis is true. A small observed p value indi-
cates that the chance of observing data as extreme as the sample data (if the null
hypothesis were true) is so small that the null hypothesis should be rejected.
Rejecting the null hypothesis is known as a strong conclusion. For example,
we have disproven that the means are equal. When we fail to reject the null
hypothesis, then the means may be equal or may not be; we really can’t tell
based on the data. This is considered a weak conclusion, for obvious reasons. If
the calculated p value is close to the specified cutoff (usually 0.05), we may
want to collect larger samples to further test the hypothesis. See also the discus-
sions of power and alpha error found in Chapter 6.

individual-X and moving-range Charts

Individual-X and moving-range charts are a set of control charts for variables
data. The individual-X chart monitors the process location over time based on
a subgroup containing a single observation. The moving-range chart monitors
the variation between consecutive subgroups over time.

When to Use

Measure Stage
• To baseline the process by quantifying the common-cause level of varia-

tion inherent in the process

Analyze Stage

• To differentiate between common and special causes of variation

Improve Stage

• To verify the results of the process improvement on the process metric

Control Stage
• To monitor the process to ensure the stability of the revised process and
the continued benefit of the improvement