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Why Control Chart Your Processes?

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  • Six Sigma Quick Poll
    How do you construct control charts?
    By hand
    Using software
    Have not constructed one yet
    No sure, Black Belt (or other) does them
    Discussion Forum
    "A stable process does not necessarily mean that it is the best performing or ideal process. If the process can be improved to an extent that only 'natural variability' remain, then it could be considered as the ideal process."
    Stable Process
    Download Products
    By Kerri Simon

    Control charting is a tool used to monitor processes and to assure that they remain "In Control" or stable. Proposed by Walter Shewart in 1924, control charts help distinguish process variation due to assignable causes from those due to unassignable causes.

    Table 1: Types Of Process Variation Found On A Control Chart
    Types Of VariationVariation Characteristics
    Assignable Cause, also known as Special Cause
  • Meaningful factors of process; not always present
  • Cause can be avoided and should be investigated
  • Not normal to process
  • Unassignable Cause, also known as Common Cause or Chance Cause
  • Factor caused by chance; always present
  • Unavoidable and inherent in a process
  • Normal and expected within process
  • Elements Of A Control Chart
    A control chart consists of:

    1. a central line,
    2. an upper control limit,
    3. a lower control limit, and
    4. process values plotted on the chart.

    If all process values are plotted within the upper and lower control limits and no particular tendency is noted, the process is referred to as "In Control." If the process values are plotted outside the control limits or show a particular tendency, however, the process is referred to as "Out Of Control" (see red-circled data points in Figure 2 below).

    Figure 1: "In Control" Process Control Chart
    In Control Process Control Chart

    Figure 2: "Out Of Control" Process Control Chart
    Out of Control Process Control Chart

    There are many types of control charts. In a future article, we will investigate the different types of control charts by process values and purpose.

    In a control chart, control limits are calculated by the following formula:

    (Average Process Value) (3 x (Standard Deviation))
    where the standard deviation is due to unassigned process variation only.

    Constructing A Control Chart
    Here is the general process for making or constructing a control chart for your process:

    1. Select the process you would like to chart
    2. Determine your process sampling plan
    3. Collect data from your process
    4. Calculate the control chart specific statistics
    5. Calculate your control limits
    6. Construct your control chart [software options]

     
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