1. Variable Control Charts
Control charts based upon measurements of quality characteristics are often found to be a more economical means of controlling performance than control charts based upon attributes.
The two most common types of variables control charts are the x-charts and R-charts.
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2. x Chart Used primarily with “variable" data
Measurements such as the concentration of hazardous materials in the air, the number of days it takes to return from an accident or the number of accidents per period.
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3. X-Chart Formulas UCL = X + (A2) * R) LCL = X – (A2) * R) Where:
X represents the average for the measurements
A2 represents the table value for A2
R represents the average range for the measurements
The row used in the table is the number of observations per sample (ie: 4 observations per week)
UCL represents the Upper Control Limit
LCL represents the Lower Control Limit
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4. Interpreting x Charts To determine if the process is in control
One point outside the 3 sigma control limits Eight successive points on the same side of the centerline
Six successive points that increase or decrease
Two out of three points that are on the same side of the centerline, both at a distance exceeding 2 sigmas from the centerline
Four out of five points that are on the same side of the centerline, four at a distance exceeding 1 sigma from the centerline anomalies
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5. Figure 9: Sample X Chart
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