Non Functional Requirements should be sticky: Quality, Usability, Performance and Capacity

Non Functional Requirements should be sticky. I argue that Quality, Usability, Performance and Capacity are the three you must keep an eye on as a priority. These define the success of any product including software applications.

The application must be tested. The tests have to be automated because otherwise the quality cannot be guaranteed over time as the amount of features to be tested goes up. Dr. William Edwards Deming philosophy for quality control can be summarized in the below ratio which should be interpreted as: Quality increases when the ratio as a whole is higher, not when the focus is just in eliminating cost. If you focus just on cutting cost most likely you are pushing problems for a near future when rework will be needed in order to correctly fix your product.

Quality = Result of Work Efforts / Total Costs

view raw gistfile1.txt hosted with ❤ by GitHub

The application must be user friendly, it must do what the user expects with the minimum user effort. Any extra mouse action, voice command, keyboard hit does matter. Usability matters.

The application is supposed to wait for the user, not the other way around. Performance matters.

The application must handle the expected load. How many users are expected to hit the new feature? Will the traffic be spontaneous because of a mailing campaign? Do not assume your system will handle any load. Do your math and avoid surprises. Capacity matters.