1/8/2011

Control Charts

1. Control Charts

A control chart includes the process data series and three (3) lines reflecting the data average, lower and upper control limits. to build a control chart, we need to build a trend chart of the process data then add horizontal lines to represent the average, lower and upper control limits.

The Add Horizontal Line  module shows us how to add individual horizontal lines. We simply need to follow this procedure for the average, lower and upper control limits to produce our control chart.

2. Organizing  Control Chart Data

To build our control chart, we need to define the start and end points for our three lines (average, lower and upper control limits).  Some users add three series next to the process data for average, lower and upper limit values.  This is ok, however it is not necessary because we can define a straight line by two points. Why add hundreds of points when we only need two?

The alternative to adding the extra points is to build a simple summary table that includes start and end dates for the three data series, like the  example to the right. This organization is simpler, cleaner and easier to understand than the multiple point version above.

When we are dealing with many data series,  mixing data with average, lower and upper limits with the trend data unnecessarily complicates the data organization.