Purpose
The Individuals Chart is used to observe and evaluate the behavior of a process over time and take corrective action if necessary. The Individuals Chart (I-Chart) plots the Individual values of each of a number of samples. The I Chart is usually plotted in conjunction with the MR (Moving Range) Chart.
Anatomy
SPC Individuals Control Chart
Reference: Statistical Process Control – Ford/GM/Chrysler
Terminology
A. Upper Control Limit (UCL) – The upper range of process control. The UCL is by convention calculated to equal to the process Mean plus three Standard Deviations.
B. Center Line – Calculated as the process Mean over the period being investigated
C. Lower Control Limit (LCL) – The lower range of process control. The LCL is by convention calculated to equal to the process Mean minus three Standard Deviations.
D. Plot of process sample statistic in chronological order vs. sample number. Any excursion in this plot above the UCL or below the LCL represents an out-of-control condition and should be investigated.
E. Out of Control Point – A single process point showing the most obvious sign of an Out-of-Control situation, i.e. being beyond either the UCL or LCL.
Major Considerations
The Individuals Chart (I Chart), together with the MR Chart, is a sensitive control chart for identifying assignable causes of product and process variation.
Application Cookbook
Purpose
To observe and evaluate the behavior of a process over time, and against control limits, and take corrective action if necessary. The MR Chart plots the moving range between individual process output values, generally the difference between two successive readings.
The MR Chart is used to visualize the process dispersion, and is usually plotted in conjunction with the I Chart (Individuals Chart), which is used to visualize the process location
Anatomy
SPC Moving Range Control Chart
Reference: Statistical Process Control – Ford/GM/Chrysler
Terminology
A. Moving Range – The Moving Range values as calculated from sequential, or chronological, readings from the process
B. Observation Number – The chronological index number for the individual moving range value being referenced
C. Lower Control Limit (LCL) – Line and numerical value representing the lower limit of the variation that could be expected if the process were in a state of statistical control, equal to the average Moving Range over the period, multiplied by a conversion factor.
D. Average of the Moving Range– Average value of the individual Moving Range values, over the period of inspection being referenced
E. Upper Control Limit (UCL) – Line and numerical value representing the upper limit of the variation that could be expected if the process were in a state of statistical control. It is equal to the average Moving Range over the period, multiplied by a second conversion factor, different from the one used to calculate the LCL.
F. Plot of the Moving Range values vs observation number. Any excursion in this plot above the UCL or below the LCL represents an out-of-control condition and should be investigated
Major Considerations
Moving Range values are correlated, because each successive point has a point in common with the preceding one. Care must be exercised in interpretation.
There will always be one less Moving Range value on the Individuals Chart.
Application Cookbook
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