Just like there are many types of discrete data charts available, there are also many types of continuous control charts available. The Six Sigma process methodology prescribes which chart must be used when. The prescription in the case of continuous data points is largely based on sample size.
Below is the list of the different control charts that need to be used for different sample sizes:
Individual Chart: In the case of an individual data chart, each observation is plotted as a separate data point. This means that there is no rational subgrouping of data. This chart must therefore be used when the sample size equals 1.
Moving Range Charts: Unlike the individual data chart, the moving range chart plots the difference between two data consecutive points. The sample size of this type of control chart is therefore 2. Here the rational subgrouping of data points is based on whether they are consecutive. The moving range data charts therefore have one less data point as compared to the individual charts.
X Bar R Chart: The X bar and R chart are used to control a process when the sample size is small and constant. This is because in small sample sizes, the range of the sample approximates the standard deviation, which would be pointless to calculate, given the small size.
The X bar and R Chart are actually two different charts, which need to be looked in tandem to understand the behaviour of a process. The X bar chart shows the average performance of the process. The R chart which plots the difference between the smallest and largest value explains the variability of the process.
X Bar S Chart: The X bar and S charts are used when the sample size used to conduct the study is large and/or variable. As in the case of X bar and R chart, X bar and S charts too have a number of values. The X bar chart ensures that the mean of the process is in control whereas the S chart monitors the standard deviation. When used in tandem, they help monitor the performance of a very large process with relative ease.
