Control chart

Control chart

1. (Shewhart control chart)

2. Seven Quality Management Tools

   • Process flow diagram
   • Check sheet
   • Histogram
   • Cause and effect diagram
   • Scatter diagram
   • Pareto diagram

3. Introduction

   • A control chart is a graphical comparison of a process performance data, computed to control limits.
   • The process performance data usually consist of groups of measurements selected to regular sequence of production while preserving the order.

4. Basic features

   • A typical control chart consist of a central line (control line or mean value) and a pair of control limits on either side of the central line known as UCL and LCL.
   • Values of a characteristic from rational subgroup are plotted on the chart.

5. Use of a control chart

   • The main use of the control chart is to detect variation of “Assignable causes "in a process.
   • The control chart distinguishes between random causes and assignable causes of variation through its choice of the control limits.
   • A control chart can be used to check
     • whether a process is in a state of statistical control (SPC) or not
     • to maintain a process which is already in statistical control.
     • Control limits are calculated from laws of probably .
     • For most control charts the control limits are calculated on the basis of average ± 3 times standard deviation of the statistic used.
     • The use of ± 3 sigma limits means that if only random causes alone are present 99.73% of the charted values will fall within control limits. The remaining 0.27% are false alarms.

6. Causes

   • Random causes
     • occurrence by chance (power failure, flood, etc)
   • Assignable causes
     • special causes that occur due to various issues (quality of raw material, work environment, etc). Assignable causes are also known s “special causes” . An assignable cause is an identifiable, specific cause of variation in a given process or measurements. A cause of variation that is not random which does not occur by chance.

7. Sigma levels of a normal distribution

8. Type of control charts

   • Control chart for variables
     • X̄ - Chart
     • X – Chart
     • R – Chart
     • σ - Chart
   • Control chart for attributes

9. Importance of control charts

   • Even though all points fall within the control limits, lack of control may be indicated by runs of seven or more points in succession on the same side of the central line.
   • Even if at least 10 out of 11 consecutive points, or 12 out of 14 consecutive points or 14 out of 17 consecutive points or 16 out of 20 consecutive points occur on one side of the central line are considered as abnormal.
   • 10 -11, 12-14, 14-17 and 16-20