Statistical Process Control (SPC)
SPC helps to
monitor and control
a
process.
Monitoring and controlling the process
ensures that it operates at its
full
potential
.
At its full potential, the process can
make as much
conforming product
as
possible with a minimum waste
Products conforming to specification are
acceptable products

Statistical Process Control (SPC)
Two phases of SPC
Understanding the process variation
Monitoring and Controlling
Finding the trend
Too early
Too late
Key factor is the cause of variation:
Common Cause or Special Cause

Statistical Process Control (SPC)
Common Cause or Special Cause
Help us in understanding when and when
not to take action.

Statistical Process Control (SPC)
Few Causes
Each Having Significant Impact
Economically viable to eliminate
Many Causes
Each Having minimum Impact
Un-economical to eliminate
SPECIAL CAUSES
COMMON CAUSES
Also Called:
Random, Chance,
Non-assignable
Also Called:
Signal, Systematic,
Assignable

Selection of Variables
What are you interested in?
Is this an important characteristic leading
value to client?
Is client specifically asking to control a
particular variable?
Is this the most difficult to maintain?

Statistical Process Control (SPC)
Univariate Control Charts
Individual chart for key characteristics
Multivariate Control charts
Monitoring multiple parameters on a single
control chart
T² Hotelling method is used to generate
multivariate charts.
Details of multivariate are not part of this
course

Rational Subgrouping
Subgroups
In control chart we select some number of
units (say 5 units) each time, summarize the
variable and plot it on the control chart.
For example in X-bar, R chart, we calculate
the mean and range of these subgroup
variables.
Subgroup is the snapshot of the process
at that time.
Measurements within a subgroup must
be taken close together in time but still
be independent of each other.

Rational Subgrouping
Two types of variation:
Within subgroup:
the variation within
subgroups is because of common cause
variation.
Between subgroup:
the variation
between subgroups is caused by special
causes.

Rational Subgrouping
Control Limits (UCL, LCL) are calculated
based on variation “within” subgroups.
Subgroup should be from single stable
source. Too much variation in subgroup
will lead to too wide control limits.
Subgroups should be time based and
not randomly selected from a already
produced items. This is a time snapshot
of the process.

Rational Subgrouping
Subgroup size:
Subgroup size of 5 is more common
Small subgroup size > meaningful
process shifts may go undetected.
Too large subgroup size > insignificant
process shifts gives false alarm