Tag Archive | Control Chart

How to Set-up a Control Chart?

1.Select appropriate type of control chart to be used
2.Gather data to establish the control chart.

  • A minimum of 30 subgroups is required over a time frame as determined by the sampling plan.

3.Plot the data in time order on a Trend Chart.
4.Compute the control limits & plot them on the trend chart
5.Outliers identification & exclusion

  • Exclude the Out-of Control (OOC) points or outliers for which there are verified/confirmed special causes from the chart
  • Re-compute the control limits, excluding the OOC points
  • If there are fewer than 30 points remaining at any time, collect more data.  It’s very important that the control limits are calculated using at least 30 subgroups.

Note:Refer to Appendix A for Control Charts for Limited Production, i.e. < 30 subgroups.

6.Validate the computed control limits against data collected by re-plotting the control chart with data & new control limits

  • Do the limits detect known problems?
  • Are the limits too sensitive?  Would they flag problems you do not know how to react to?

7.Use the control limits established to monitor the critical parameter identified
8.For each parameter, every machine should have a separate control chart with separately computed control limits

What is a Control Chart?

  • A trend chart with control limits
  • Graphical representation of process performance, where data is collected at regular time sequence of production
  • Valuable tool for differentiating between common cause and special cause variation
  • Evaluating whether a process is or is not in a state of statistical control
  • It lets the data ‘talk’ by itself & basis for data-driven decisions

Control Limits
A typical control chart consists of three lines :

control chart

control chart

CL: The average (measure of location) process performance when the process is in-control

UCL & LCL: The range of ‘usual’ process performance when the process is stable.  Lines drawn 3 standard deviations (3 sigma) on each side of the center line.