For more info about the idea, see Pareto principle on Wikipedia.

Here is a list of possible applications of the principle to software development:

• Implement only top 20% features
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• Focus on the top 20% systems bringing the most business value
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Research:

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• Study the basics of science and math before higher-level theories

Julian Shapiro argues that the 80/20 rule works best for mid-term planning.

Related:

• Always work on the highest-leverage things
• Strategies for moving between projects and focus areas
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• Say "No" to many things (to almost everything)