The 80/20 Rule

The 80/20 rule, also known as Pareto’s Principle, states that 80% of an observable effect is caused by 20% of the variables at play. The first recognition of this rule was by Vilfredo Pareto, who in 1906, recognised that 80% of Italy’s wealth was owned by 20% of Italy’s population.

The same 80/20 split can be seen in many places; management, economics, software/interface design, quality control systems and many more places. Whilst the rule is called the 80/20 Rule – the actual percentages are not all that important: 90/10, 70/30, 80/20 are all valid variations. Indeed – it is perfectly valid to say 70/10 as there is no requirement that the two sides add up to 100 – the important thing is the principle that a large number of outputs are determined by a relatively small number of inputs.

So using the 80/20 rule we can say:

  • 80% of users only use 20% of features
  • 80% of bugs are caused by 20% of the code
  • 80% of development time is spent on 20% of the code

This rule of thumb is useful when focusing our time and efforts on improving our products. We first need to find out what our most important 20% is. The easiest approach to this is to simply ask customers what their favourite features are or maybe observe them using the product. Developers of web applications could use tracking software such as Google Analytics to provide detailed information on which pages customers are accessing the most. Alternatively, simple logging could be added to a desktop or web application.

Armed with this information we will know which 20% of features provide 80% of the value of our products. We can then spend time polishing these features rather than diverting efforts on to rarely used features. This approach will lead to the greatest improvements and make the biggest contributions to customer satisfaction. If we make a change that benefits 80% of our customers then that’s a big win and is going to generate a lot of good feelings. Much more so than adding a similar scoped feature to an area that falls under the less used category.

This isn’t to say we should completely neglect the less frequently used features – that proportion is likely to be very important when it is needed – but when we are faced with limited time and resources, focussing our efforts on the most used 20% will gain a disproportionately large benefit to our product and users. It is rarely considered a bad thing by users when we make improvements to the benefit of our most used and popular features!

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