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Xbar and s chart example12/22/2023 If the X-bar chart indicates the process is “out of control”, i.e., that Special Cause is present. X-bar chart and Special Cause If the R-chart is in control, i.e., stable and predictable, then any shifts in the mean of the process come from Special Cause. (More on this later in Rational Subgrouping.) If no signal of Special Cause is indicated, this is not proof that within subgroup variation is only due to Common Cause.The chart should not indicate the presence of Special Cause. If the data in each subgroup was collected under “homogeneous conditions”, then the Ranges should reflect only Common Cause.In Quality Control, we use the “1 out of 100” criteria to say that our process has more variation than just Common Cause (idea due to Shewhart, it is simple but effective in practice).The chance of falsely declaring a treatment difference exists is then about 1 out of 20.In hypothesis testing we say there is a treatment difference if pthe probability that the chart will falsely indicate the presence of Special Cause is about 0.01. If only Common Cause is present in the process, then the chance of a false signal is about 1%, i.e.Indicate when you are reasonably sure that Special Cause is present.Make sure that the operator does not react to Common cause.Table 3.2 constants for X-bar and R charts Raw data plot, thickness vs sample number (everyone else has gone metric but…….)ĭata structure Sample Value 1 1 1 4 1 6 1 4 2 3 2 7 2 5 2 5 3 4 3 5 3 5 3 7, etc. Data on part thickness Thickness of parts recorded as amount by which thickness exceeded 0.300 in.
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