Pareto diagram-a pareto diagram is used to rank the importance of a problem based on its frequency of occurrence over time. this diagram is based on the pareto principle, which is more commonly referred to as the 80/20 rule. applying the principle to quality control, it says that the majority of the project defects are caused by a small set of problems. a pareto diagram helps to isolate what the major problems are, so that you can take action that will have the greatest impact. a bar graph is used to display problems in decreasing order of occurrence so that priorities for improvement can be established.