Control Chart, Statistic control, common and special cause variation

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Control Chart


The control chart is a graph
used to determine  process variation over time.
A control chart always has a central line for the average,
an upper line for the upper control limit and
a lower line for the lower control limit.
These lines are determined from historical data.











Statistic Control

Output is within the control limit or absence of point beyond the control limit
        then process is said to be statistical control
Or Special cause / Assignable * cause have been removed from process
        then process is said to be statistically controlled process.



The variations in process can classified into common cause and special cause variations

Common cause variation ( chance cause variation ) : this variation occur due to factors which are inherent in the process
e.g. normal tool wear

Special cause variation  (assignable cause variation) : this variation occurs due to change in external factors such as change in raw material, change in operator, change in tools and equipment used etc


 Any process that has assignable cause variation is said to be out of control process

 other Examples of special cause variation
  • Poor adjustment of equipment
  • Operator falls asleep
  • Faulty controllers
  • Machine malfunction
  • Fall of ground
  • Computer crash
  • Poor batch of raw material
  • Power surges
  • Broken part
  • Lack of awareness
  • Operator absent[4]
 


Q. What do you mean by a control chart ?
Q. What is statistical control ?
Q. What is out control process  ?
Q. What is common cause variation and special cause variation ?

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