Pareto Principle- 80–20 rule of automation testing

80-20 rule that says that 80% of the results are coming from 20% of the subjects.
 
This is a good rule of thumb for determining how much testing is enough. 
you can typically test approximately 80% of the functionality that people actually use by focusing on 20% of the possible test combinations.

This can be applied to any field as follows:

- 80% of the revenue of a company is coming from 20% of the clients

- 80% of the donations for a charity are coming from 20% of the people

- 80% of the books from a bookstore are purchased by 20% of the clients

- 80% of the clients are using 20% of the functionality

- 80% of the bugs are caused by 20% of the functionality

Microsoft noted that by fixing the top 20% of the most-reported bugs, 80% of the related errors and crashes in a given system would be eliminated
In load testing, it is common practice to estimate that 80% of the traffic occurs during 20% of the time

How does this apply to testing?

Well, the project release date is fixed so you cannot test everything well.

So, test only 20% of the application as this is what the majority of the users will use.

Select the 20% of the application's functionalities that have the highest risk and test them well.

Test the remaining 80% of the functionalities by just taking the happy paths.

You think you did a good job, the project manager is happy with the results.

And after the release, the support team receives lots of issues from the clients about the 80% of the application not tested well.

More, the senior management of the company starts noticing problems all over the application too.

The solution is, of course, applying an endless number of patches with bug fixes for the issues discovered by the customers, frustrating the customers as much as possible and wasting as much time as possible for both the development and testing team.