Uses statistical sampling and known as statistical process control.
take quality measurements of different samples. Find the average quality. Draw a line with the average quality. Plot all the samples's quality. Draw the line for upper quality limit. Draw the line for lower quality limit. If all/most points are in between the process may be in control. If seven consecutive samples fall on one side of the average line the samples must be inspected.
Shows how different components relate and determine where quality problems may occur. Example: Cause and effect diagram