eighty/twenty rules

wealth Italian economist Vilfredo Pareto discovered that, in Italy, 80 percent of the country’s wealth was held by 20 percent of the country’s population. The 80/20 rule was attributed to him.
measuring progress After spending about 20% of the time, many programmers claim that they have completed 80% the work. Later reality strikes from the back. Upon having spent about 80% of the time the same programmers come to realize that they have resolved only 20% of the problem.
efficiency of tools Software development tools like 4GLs and report-writers solve 80% of the problem in 20% of the time. The remaining 20% of the problem however continues to take the other 80% of the available time.
scope of a system This looks a lot like the previous statement. Software systems that are designed to ignore the last 20% of the reality, cause 80% of the user's time getting spent on resolving the missed features.
Theo van Eijndhoven, 2004