The control chart is a graph used to study how a process changes over time. Data can be plotted in time order. A control chart has a central line for the average, an upper line for the upper control limit, and the lower line for the lower control limit. These lines are determined from historical data. 

For variable data, control charts are used in pairs. The top chart monitors the average or the centering of the distribution of data from the process. The bottom chart manages the width of the distribution and the range of the distribution. They are also called Shewhart charts and statistical process control charts. The action has to be taken to correct the variations that negatively affect the company; here the control chart will come as an important part. 

CONTROL CHARTS-

As discussed above, control charts give the statistical test to determine if the given set of data presents convincing evidence if the process has changed or not. 

The main elements of the control chart include-

• A visual time-series graph tells about the data points collected at specific time intervals.
• Horizontal control lines can easily visualize variations and trends.
• Horizontal lines represent upper and lower control limits and are to be placed at equal distances above and below the control line. These upper and lower limits can be determined from the data on the time-series graph. 

BENEFITS OF USING CONTROL CHARTS-

Control charts are used for –

• Understand the variation. If the process is working or not can be determined if the variations are within the control limits. 
• To determine the pattern within plotted points. The patterns indicate the causes, which will help find possible solutions. 
• Help predict future performance. 
• If something goes wrong then it will ask you to take measures to rectify it. 

CONTROL CHARTS NOTES-

The control chart analyzes process performance by plotting data points and control limits. The objective of a control chart is to monitor the performance and control the adjustments when required. Regular monitoring of the process will help in preventing unnecessary inspection and adjustments. Control limits are calculated by the formula, they are the voice of the process. 

When to use a control chart-

Control Chart is used for controlling any ongoing processes by finding and correcting problems. They are also used in determining whether a process is stable or not. It is used to determine if your quality improvement project should aim to prevent specific problems or to make fundamental changes to the process. 

HOW TO MAKE A 3 SIGMA CONTROL CHART-

Three-sigma limits are defined as the statistical calculation. It is where the data is within the three standard deviations from a mean. Three-sigma is referred to as a process that is operating efficiently and producing items that are of the highest quality. Three-sigma limits are helpful in setting the lower and upper limits in control charts in statistics. Talking about the bell curve, data that lies above the average and lies beyond the sigma line represents less than 1 percent of data.

Steps required to make three-sigma control charts are-

• Calculate the mean of the given observed data
• Calculate the variance of the set. Variance is defined as the spread between the data points. It is calculated as the sum of the squares of the difference between each data point and the mean is divided by the number of observations. 
• Find the standard deviation. Standard deviation is the square root of variance. 
• Now, calculate three-sigma which is referred to as three deviations above the mean. 

Three-sigma limits basically set the range for the process parameter as 0.27 percent. Three-sigma limits are used to check data processes and to know if it is within the control limits. It can be done by checking that data points are within the three standard deviations within the mean. 

According to statistics, 99 % of the controlled processes occur within plus or minus three sigma. Talking about the bell curve, data that lie above the average and beyond the three-sigma line represent less than 1% of all data points.

CONCLUSION-

Above we have discussed control charts. A control chart is basically a graph that is used to study how the process changes over time. It was invented by Walter Shewhart in 1920. We have discussed the objective of control charts and the benefit of using them. We have even talked about the main elements of the control charts and the usage of the control charts. The control chart is used to determine if a manufacturing process is in a state of control. Previously, the control charts were designed such that they can monitor process parameters. Nowadays, distribution-free control charts are popular control charts. We have even talked about the step-by-step process for calculating the three-sigma control charts.