We do create different type of charts to fulfill our charting needs – quite a few of them are related to quality control purposes. In this post we will learn how to plot a control chart for mean of a value i.e. value of a mean for some measureable quantity over time.
A little bit of theory on Statistical Quality Control:
Control chart is a statistical quality control technique to monitor the variability of a variable over a period of time. We usually monitor on two different lines for quality control:
- Monitoring Variables
- Monitoring Attributes
For this post, we will be studying charts for monitoring variables – X - Charts. X-Chart simply monitors the mean of a variable. The mean value in Statistics is represented by X, hence called mean charts.
In order to calculate where should we draw the boundaries…
We need some formulas from the statistics textbooks to proceed. Using Montgomery’s all known book on Statistical Quality Control (or just goggling it on web), we can have these formulas:
With all this information in hand we can easily plot an X chart
In the attached sample sheet we have the sample data.
We first need to calculate the sample means, and they are calculated by averaging the values in rows. For first sample we will average out and the values in for C4:G4 and will report it in H4. The formula we will use is: