Process Capability Flashcards
Name four characteristics of shor-term variation (sst).
Represents the process capability
Captures variation due to common causes
Measures variation within a subgroup or between successive values
Used for calculating control chart limits
Name four characteristics of long-term variation (slt)
Represents the total process variation
Captures variation dues to common AND special causes
Measures variation in all data
Should not be used for calculating CC limits
Which indices address short term capabiliy?
Cp and Cpk
Which indices address long term capabiliy?
Pp and Ppk
Calculation of Cp
=(USL-LSL)/6 sst
What are four characteristics of the Cp value?
The average is not part of the formula
A measure of potential “best-case” process capability if stable and on target
Can be misleading if process is unstable or off target
Must have both a low LSL and USL to calculate
Calculation Cpk
= (min(USL-avg, avg-LSL))/3sst
What are four characteristics of the Cpk value?
Cp=Cpk if process is on target
Still a measure of potential capability if the process is stable
Can be used for 1-sided specifications
A negative cpk is possible if the average is outside the specifications
Calculation Pp
=(USL-LSL)/6slt
What are three characteristics of the Pp value?
Considered a “Process performance index”
If stable : Cp=Pp
Can be used misleading if process is off target
Calculation Ppk
= (min(USL-avg, avg-LSL))/3slt
What are seven characteristics of the Ppk value?
If stable, Cpk = Ppk
If on target, Pp=Ppk
If stable and on target, Cp=Ppk
Can be used for one sided specs
Best indicator of actual process performance
A negative Ppk is possible if the average was outside specifications
