Statistical Process Control (SPC) and Control Charts in Six Sigma Flashcards
Are all processes subject to variation?
Yes.
Define Statistical Process Control
A way to measure, monitor, and control processes.
SPC is a methodology that uses control charts to determine when a process is out of control.
Describe the larger “continuous improvement” philosophy undergirding SPC
Continuous improvement
Inputs > Process/System > Outputs
SPC analyzes the “Process/System” part
Inputs are: People Materials Methods Equipment Environment Measurement
Output = Product/Service
Define common cause variation
Those causes that are inherent to the process and not controllable by process operators.
Define special cause variation
AKA “assignable causes”, include unusual events that the operator can usually remove.
Do the effects of common cause vs special cause variation need to be similar?
No. They often are not.
It is the CAUSES that vary.
What are the three objectives of SPC?
- Monitor the performance of a process
- Identify special and common cause variation
- Control and improve process performance
Describe (1) “monitor the performance of a process” objective of SPC
- Determine process capability and the natural range of the process
- How the process measures up to specifications
- Charts: control, histogram, run charts, check sheets
Describe the difference between “defect” and “control.”
A defect is determined by the process violating the specification limits.
A process would be call unstable/out of control when it violates the control limits/exhibits abnormal behavior.
Describe control limits vs specification limits.
Control limits look inward and are calculated from process data itself.
Specification limits are usually determined by factors external to the process (customer requirements, industry standards, etc).
List the considerations for implementing SPC
- Team members’ buy-in
- Avoiding overanalysis
- Focus on process issues (not human ones)
- Feedback on process behavior
- Control charts
Rational Subgrouping
A small homogenous sample from the process taken in a short space or time, such that every item in the subgroup is produced under similar conditions.
Done so that just normal/random effects are within that group.
Two types of variation
Within subgroup and between subgroup
Minimize within subgroup variation, maximize between subgroup variation.
Rational subgrouping principles
- Observations must be independent
- Obersvations are from a single, stable process
- Observations taken in a time-ordered sequence
Variation of interest
Choose rational subgrouping in a manner that isolates your variation of interest. Ex:
- Shift to shift
- Day to day
- Hour to hour
- Batch to batch
Selecting variables for SPC
Use your judgement, but can generally choose variables that:
- Most difficult to hold
- Tied to customer, organization, or regulatory imperatives
- Represent a critical dimension of the product or process
- Salient or known
Selecting variables for SPC
Use your judgement, but can generally choose variables that:
- Most difficult to hold
- Tied to customer, organization, or regulatory imperatives
- Represent a critical dimension of the product or process
- Salient or known
Most difficult to hold
Directly associated with high defect rates
Known to exhibit a lot of variation
Tied to customer/organizational/regulatory imperatives
- Customer complaints
- Key customer requests
- Standards
Variables representing a critical dimension of the product/process
- Those that effect human safety, environment, or the community
- KPIVs, KPOV (key process input/output variables)
- Variables that have caused processing difficulties
- Variables that help control the process
- Variables that contribute to high internal cost
The Ideal: one variable that’s so critical, you HAVE to do it
If you CAN’T tell, use other tools to narrow down
- analysis of variance
- Pareto analysis
Salient/known variables
- Variables with special cause variation
- Variables that can be measured
- Items that can be counted by the person counting
- A leading indicator
How to use these categories in your own project?
Narrow down to those with biggest effect on:
- Cost
- Customer satisfaction
- Key process variable
Avoid overcharting
When you use more than a few charts, the benefits increase costs, not decrease.
How to apply rational subgrouping
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