How Do You Calculate Upper & Lower Control Limits?
A control chart is a chart used to monitor the quality of a process. The upper and lower control limits are two horizontal lines drawn on the chart. If data points fall outside of these lines, it indicates that it is statistically likely there is a problem with the process. These lines are usually placed three standard deviations from the mean, so there is a 99.73 percent probability that a data point will be within those limits. To calculate the control limits, you will need to first find the mean and standard deviations of your data set. Then you will calculate the upper and lower control limits. Find the mean of your data set by adding all the data points and dividing by the number of data points. As an example, take the data set: 2, 3, 5, 5, 7. The mean is 2+2+3+5+5+7 / 6 = 24 / 6 = 4. Subtract the mean from each data point and square the result. Continuing the example: (2-4)^2, (2-4)^2, (3-4)^2, (5-4)^2, (5-4)^2, (7-4)^2 = (-2)^2, (-2)^2, (-1)^2, (1)^2, (1)^2, (3)^2 = 4, 4, 1, 1,
