Statistical process control is a specific approach to monitor the operational process in order to identify specific variations. The process of SPC is gathering of data about a process, the use of control charts, and establishing feedback loops to bring a process into statistical control, and then keep the process in a state of statistical control. The most commonly use SPC charts are mean and standard deviation chart, mean and range charts, and median and range charts.
1.1 common and special cause variation
During the production, two causes can common and special variations. Common variations are inherent in the process and will remain until management sees fit to change the process in order to modify or eliminate them. Thus, it are random or natural source of product variation, and unassignable. Special cause variations are caused by external factors that can be eliminated or control, and these external factors aren’t a part of the process such as incorrect packaging material, improper identification of raw material. Thus, statistical process control is used to control the special cause variation.
1.2 What’s mean and standard deviation
Mean is the average of a distribution, and standard deviation is a measure of the average deviation of data values from the means. Thus, mean and standard deviation can be used to calculate upper action limit, upper warning limit, mean line, lower warning limit, and lower action limit by getting data from the production. In general, people should take a minimum of 25 samples, including between 4 and 16 consecutively produced items during the production to enable an estimate of good process spread and location.
1.3 how to set up the mean and standard deviation chart
* calculate means and S for each sample
* calculate the Grand Mean and Mean Standard Deviation for the sample set
* use the appropriate coefficients and the equations to calculate the control limits
* to show the control limits and each data from each...