Assuming that your processes produce data on a very low frequency, maybe once a week or twice a month, how long do you think will it take to collect several data elements? Certainly, a lot. As a result, you would rarely plot a point. In this type of situation, individuals control charts should be used.
What Is The Individuals Control Chart?
The Individuals control chart helps in examining variation in the data elements. The chart is useful when you have just one data point at a given time to depict a situation. Like other variables control charts, it contains two charts:
Chart1: Represents Individual Sample Result (X). The chart indicates long-term variation in the process or how the mean or average changes over time.
Chart2: Represents Moving Range (R) between consecutive individual samples. The chart indicates short-term variation or how the range of subgroup varies over time.
“Usually, the X-Bar Chart is combined with an R-Chart to inspect processes where data is not available frequently. The chart monitors variation in sample results over time and checks whether the process is in control.”
-Sanket Saha, Process Manager.
5 Benefits of Using Individuals Control Chart
Benefit#1: Individuals control charts can be used when you have a single data element to represent a given situation. You can plot an element on the chart for each sample collected. This allows you to establish whether or not your process is in statistical control for each sample collected.
Benefit#2: Control charts (X-Bar and R Charts) are useful in a manufacturing or production environment to control, examine and improve a process. Common variations arising due to material, machines, time, etc. can be corrected and normal output can be resumed.
Benefit#3: While it is not possible to subgroup data elements due to low frequency, you can use the control chart for the assessment of results.
If your processes are in statistical control (indicating the consistency in individual sample results or successive sample results over time), the average on the Individuals chart is your population average that can be used to calculate the population standard deviation.
Moreover, since the processes are in statistical control, you can predict how the processes will perform in the future.
Benefit#4: The objective of plotting the control chart is to identify any variation in the process. If the data elements are exceeding the control limits, the operator would immediately adjust.
Benefit#5: The chart can be used to identify statistically unnatural patterns and decide when to take action.
“If you are thinking of using the Individual Charts more often, you should first ascertain if the individual sample results are normally distributed. If not, the chart is likely to give more false forecasts for the tests, indicating that your processes are out of control when they are not.”
- Pankaj Tyagi, Quality Control Specialist.
How To Use?
Select the suitable control chart for your processes.
Determine the suitable time period for gathering and plotting data.
Gather data, plot your chart, and examine the data.
If you find “out-of-control signals” on the chart, investigate the cause. Record how you identified the cause and rectified it.