Design of Experiment
Where you are analyzing some quality problem and you are looking for various sets of variables and there impact of output you use design of experiment... Say the problem involve 4 variables lets name then , soil ratio, cement ration , water ratio , iron , now you are trying with some set of soil , cement , water and iron and checking how this impact the output. When we change all these elements (in pattern) and try it is called design of experiment.
Lets say you have made rule that all workers should come at 9:30, this rule you made based on historic data since you see most of the people come at 9:30, now in this you put higher and lower control limits... like 9:20 and 9:40 , so workers comes between 9:20 to 9:40 the process is under control... if they come before 9:20 or after 9:40 it requires your attention since system is going out of control...
The rule of seven can be visualized as, is someone consistently coming at 9:30 for 7 days..then also you need to talk to the person since he can any time breach the 9:40 mark since his mean is not at 9:30.