Control Charts
Control charts are used to determine whether process stability has been upset by special or assignable causes. Oracle Quality provides the following types of control charts and pairs of control charts.
- Xbar and R charts (XBar R): A pair of line graph charts which plot subgroups of data collected over a continuous period of time. The Xbar chart plots the average value for each subgroup. The R chart, or range chart, plots the range of values within each subgroup.
- Individual X and Moving Range charts (XmR): A pair of charts based on individual data points instead of subgroups. The individual X chart simply plots individual data points. The moving range chart plots a specified range of the data points up to and including the current data point. For example, if a subgroup size of 5 is specified then the 10th point on the Moving Range chart is plotted using the range between points 6 and 10. These charts are commonly used when the process results do not fall into subgroups, for instance when the process is slow and data does not form clusters over a period of time.
- Xbar and S charts (XBar S): A pair of line charts similar to the Xbar and R charts except that the standard deviation is used instead of the range when the S chart is plotted. This pair of charts is commonly used instead of the Xbar and R chart when the subgroup size is large (>10) because it mitigates the effects of outlying data points. For example, if you had a subset size of 20 that contained one outlying data point, the range plotted on an R chart would be large even though the remainder of the data is fairly evenly distributed.
Subgroups
Selection of a subgroup is extremely important when defining a control chart. Improper subgroup selection can lead to misleading results and false corrective actions.
See Also
Creating and Viewing Control Charts
