The 80/20 Rule
The 80/20 rule asserts that approximately 80% of the affects generated by any system are caused by 20% of the variables or elements in that system.
80% of a website’s usage involves 20% of it’s features or products
80% of progress comes from 20% of the effort
80% of innovation comes from 20% of the people
This rule, in design, is used to determine where effort and resources should be focused. All project requirements are not equal & all website bugs are not equal. Focus on that key 20% and reassess the remaining 80% on an ongoing (iterative) basis to get the most efficient and effective (i.e., best converting) system.