When distributions of averages arecompared to distributions of individualvalues, the averages are grouped closerto the center value than are the individualvalues, as described by the central limittheorem.
What does this imply for averages incontrol limits versus individual values inspecification limits?
Process Capability
Estimation of populationsigma from sample dataEstimation of populationsigma from sample data
The relationship between the standarddeviation for individual values and thestandard deviation for their averages isgiven by the formula:
Where n = subgroup sample size
Process Capability
To simplify calculationTo simplify calculation
If the process can be assumed to benormal, the population standard deviationcan be estimated from either thestandard deviation associated with thesample standard deviation or the range:
or
Process Capability
Control limits andspecification limitsControl limits andspecification limits
Xbar charts do not reflect how widely theindividual values composing the plottingaverages spread.
The spread can only be seen byobserving what is happening on the s or rchart.
Process Capability
The Six sigma spreadversus specification limitsThe Six sigma spreadversus specification limits