Process Variation
Process Variation is the inevitable differences among individual measurements or units produced by a process.
Sources of Variation
• within unit • between units • between lots • between lines • across time • measurement error
Process Capability Analysis
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Process Capability vs Specification Limits
a)
b)
c)
a) Process is highly capable b) Process is marginally capable c) Process is not capable
said to be “in statistical control”
Process Capability Analysis
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Types of Variation
Special or Assignable Variation
• May be due to a) improperly adjusted machine b) operator error c) defective raw material
a) percentage (%) b) parts per million (ppm) c) parts per billion (ppb)
Process Capability Analysis
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Process Capability
Process Capability studies can • indicate the consistency of the process output • indicate the degree to which the output meets specifications • be used for comparison with another process or competitor
– Cycle-Time (Exponential Distribution) – Reject Rate (Binomial Distribution) – Defect Rate (Poisson Distribution)
Process Capability Analysis
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Process Capability Analysis
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– Specification, Process and Control Limits – Process Potential vs Process Performance
• Short-Term vs Long-Term Process Capability • Process Capability for Non-Normal Data
Process Capability
Process Capability is the inherent reproducibility of a process’s output. It measures how well the process is currently behaving with respect to the output specifications. It refers to the uniformity of the process.
Capability is often thought of in terms of the proportion of output that will be within product specification tolerances. The frequency of defectives produced may be measured in
• A process operating in the presence of assignable causes of variation is said to be “out-of-control”
Process Capability Analysis
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(positional variation) (unit-unit variation) (lot-lot variation) (line-line variation) (time-time variation) (repeatability & reproducibility)
Process Capability Analysis
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Process Capability Analysis
(Measure Phase)
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Scope of Module
• Process Variation • Process Capability
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Types of Variation
Inherent or Natural Variation
• Due to the cumulative effect of many small unavoidable causes • A process operating with only chance causes of variation present is