Hach ORBISPHERE G1100 User Manual page 48

Cp process capability
Cp is an index used to assess the width of the process spread in comparison to the width of
the specification. It is calculated by dividing the allowable spread by the actual spread.
A Cp of one indicates that the width of the process and the width of the specification are
•
the same.
A Cp of less than one indicates that the process spread is greater than the specification.
•
This means that some of the data lies outside the specification.
A Cp of greater than one indicates that the process spread is less than the width of the
•
specification. Potentially this means that the process can fit inside the specification limits.
CPk process variability
Cpk takes into account the center of the data relative to the specifications, as well as the
variation in the process.
A Cpk value of one indicates that the tail of the distribution and the specification are an
•
equal distance from the overall average.
A Cpk of less than one means that some of the data is beyond the specification limit.
•
A Cpk greater than one indicates that the data is within the specification.
•
The larger the Cpk, the more central and within specification the data.
•
CPm process repeatability
Capability index that takes into account variation between the process average and the target.
If the process average and the target are the same value, Cpm will be the same as Cpk. If the
average drifts from the target, Cpm will be less than Cpk.
negative
positive
Figure 30 Skewness
Figure 31 Kurtosis
Skewness
An asymmetric frequency distribution is skewed to the left if
the lower tail is longer than the upper tail, and skewed to the
right if the upper tail is longer than the lower tail. Distributions
of positive-valued random variables values are often skewed
right.
Kurtosis
Kurtosis is a parameter that describes the shape of a random
variable's probability distribution.
The graphs on the left illustrate the notion of kurtosis. The
lower curve has higher kurtosis than the upper curve. It is
more peaked at the center, and it has fatter tails
View Menu
47