3.
Production
Process Characterization
3.1.
Introduction to Production Process
Characterization
3.1.3.
Terminology/Concepts
3.1.3.1.

Distribution (Location, Spread and Shape)


Distributions are characterized by location, spread and shape

A fundamental concept in representing any of the outputs from a
production process is that of a distribution. Distributions
arise because any manufacturing process output will not yield the
same value every time it is measured. There will be a natural
scattering of the measured values about some central tendency value.
This scattering about a central value is known as a distribution. A
distribution is characterized by three values:



Location

The location is the expected value of the output being measured. For a
stable process, this is the value around which the process has stabilized.

Spread

The spread is the expected amount of variation associated with the output. This tells us the range of possible values that we would expect
to see.

Shape

The shape shows how the variation is distributed about the location. This
tells us if our variation is symmetric about the mean or if it is skewed
or possibly multimodal.

A primary goal of PPC is to estimate the distributions of the
process outputs

One of the primary goals of a PPC study is to characterize
our process outputs in terms of these three measurements. If we can demonstrate
that our process is stabilized about a constant location, with a constant
variance and a known stable shape, then we have a process that is both
predictable and controllable. This is required before we can set up control
charts or conduct experiments. 
Click on each item to read more detail 
The table below shows the most common numerical and graphical measures
of location, spread and shape.
probability plot
