Statistical Process Control - Mean and Range Charts

1. SPC can be used as a means of preventing product non-conformance 

2. Causes of variability may be categorised as common causes and assignable causes 

3. Variables are features of a process output which can be measured on a continuous scale; attributes are features which do not lend themselves to measurement, but allow the output to be segregated into conforming and non-conforming items 

4. Variability can be assessed by analysing measurements to estimate the population mean and standard deviation 

5. Samples are used, to take advantage of the central tendency of sample means i.e. the means of samples cluster more closely around the population mean than do individual measurements 

6. Mean and range charts are used to monitor process variables 

7. Process capability is assessed by comparing the process variability with the product design tolerance band 

8. P-charts, np-charts and c-charts are used to monitor process attributes.