The 80-20 rule

The 80-20 rule, also known as the Pareto principle, states that roughly 80% of the effects come from 20% of the cases. If we apply this to software development it can be rephrased into a few different rules or principles:

• 80% of the functionality will take 20% of the implementation time.
• 80% of the functionality can be implemented for 20% of the cost.
• 80% of the users only uses 20% of the functionality.
• 80% of the errors and crashes will be eliminated if 20% of the top reported bugs are fixed.

Based on my own experience I agree with all of these principles. What about you?