Six Sigma SPC - Statistical Process Control

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R-Bar - Range

Part of most of any statistical process control plan. Range is the spread of the observations, (subgroups), using stratified sampling and usually a random sampling plan.

It is calculated by subtracting the lowest sample from the highest sample in the observation set. R-Bar, is the average range of a set observations, (subgroups). This data is typically charted on a line control chart with the center line being R-Bar, (an average of the ranges), and upper control limits and lower control limits. (see X-Bar and Range, (R-Bar), Control Charts for a pictures and additional information of these types of charts)

Range measures the variability of the process.

N is the largest sample in the observation set
n is the smallest in the observation set

 R = N - n



Range1 - (.051 - .049) = 0.002

Range2 - (.052 - .049)= 0.003

R-Bar = (Range1 + Range2) / 2  = 0.0025
R-Bar = 0.0025

For an example of a R-Bar charts see our ZeroRejects features page. It makes doing six sigma and statistical process control charts easy and it only costs $150.00.

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Last Updated: Thursday, 11-May-06 12:44:32 PDT