NAMITHA M R2011-02-028B.TECH AGRL. S7KCAET, TAVANUR

CONTROL CHARTS

Definition

A graphical representation of collected information

A statistical quality control tool used in monitoring the variations in the characteristics of a product or service

One of the Seven Basic Tools of Quality

Also known as Shewhart charts or Process behavior charts

First described by Walter A. Shewhart

Purpose- To determine if a manufacturing or business process is in a state of statistical control

Used to study how a process changes over time

Consists of...

A central line for the average

An upper line for the upper control limit (UPC)

A lower line for the lower control limit (LPC)

Comparison of current data to these lines provides details about whether the process variation is:-

Consistent (in control) or

Unpredictable (out of control)

How to draw?

Points representing a statistic or data is taken from the process at different times

The mean of this statistic using all the samples is calculated

A centre line is drawn at the value of the mean of the statistic

Standard error of the statistic is calculated using all the samples

Upper and lower control limits are drawn typically at 3 standard errors from the centre line

If all data points are within the control limits, the process is said to be ‘in control‘

If data points fall outside the control limits, the process is said to be out of control

Control chart

Control Chart: Out-of-Control Signals

Advantages...

Determines whether the process is in control or not

Monitor process variation over time

Detects unusual variations taking place in a time

Differentiate between special cause and common cause variation

Assess effectiveness of change

Communicate process performance

Ensures product quality level

Disadvantages

False alarms

Flawed assumptions

Special training

Misplaced control limits

Conclusion

An understanding of statistical quality control and variation is essential for an effective assessment process

Statistical tools like control charts are especially helpful in comparing performance with historical patterns and assessing variation and stability