Among the above mentioned Non-functional characteristics, there is one characteristic which can single handedly decide the success or failure of any product.

And it is "Usability" or "Ease of Use" of the system.

Let's understand this with an example.

When it comes to Mobile phones, you must have come across your friends or colleagues saying that a particular "Brand X" is more user friendly as compared to "Brand Y".

Can you think as to why they feel that "Brand X" is more user friendly as compared to "Brand Y"?

People say:

"Brand X" is more easy to use as compared to "Brand Y"


"Brand X" is more intuitive as compared to "Brand Y"


"Brand X" is highly usable as compared to "Brand Y".

Let's first discuss as to what exactly is Usability.

"Usability is a non-functional constraint that defines the ease of use with which the end user interacts with the system".

Usability is defined with respect to the end user of the system.

What is the technical definition of Usability?


When does a System become easier to use by the end user?

In my view the following are the properties for a system to be user friendly to its end user.

  1. A system is considered to be user friendly if it is designed to be a Black Box to the end user i.e. the internal structural complexity is not visible to the user.
  2. A system is considered to be user friendly if it is designed in such a manner that for every behavior that it offers, it takes the minimum number or minimum type of inputs and gives the maximum number of relevant outputs.

The first part of the answer gives us reasoning as to why the systems are designed to be a black box. We will revisit this point again in the next chapter.

Let's try to understand each of the above.

What do you understand by "Minimum number of inputs" and "Minimum type of inputs"?

As we know every Feature/Functionality/Behavior can take one or more inputs and one or more type of inputs.

Generally it is considered that "More the number of inputs needed by a behavior of a system, More difficult for the human brain to use that behavior.

Similarly "More the different types of inputs needed by a behavior of a system, More difficult for the human brain to use that behavior

Hence as a principle; if a behavior needs minimal number and type of inputs, the behavior offered by the system is considered to be highly usable.

You must have across shortcut keys on your computers or mobile phones on the main screen of application.

What are these shortcuts? What are they meant for?

According to me, "Short cuts are means for accessing the most frequently used functionalities with minimum efforts or inputs."

Now what do you understand by Maximum relevant output?

In the definition of Usability, it is said that for a system to be highly usable, the behavior should give you maximum relevant output.

Why does it say "Maximum relevant output" and not just "Maximum output"?

Let's take an example of the search functionality of our phone book.

I type character "A" within the search text box and my phone shows me 1000 words with character "A" on the screen.

Will I consider this behavior to be user friendly?

Let's understand that showing or throwing unwanted or unrelated information to the user reduces the usability of the system.

For example, I want to search for a name like "Anand" and I am made to go through 500 names to search the same. In this case although the system has given me the maximum output, it has made the whole search process very complex.

Therefore, keeping this in mind, you see the search functionality giving results that are specific to the position of occurrence of characters in the word or based on the pattern of search characters.

The relevance of output shown to the user by a behavior is defined by the context from which the system is used or viewed.

Hence if I just say that my phone has a feature called phone book, it doesn't completely describe the system from a user's perspective.

If I have to completely define the same, I will also have to describe the inputs or types of inputs the behavior takes in and the output that it shows in response to the inputs provided by me or any user.

So let's redefine a system from a user's perspective.

"Any well designed system from a User's perspective should be a Black Box which offers a set of Features/Functionalities/Behaviors to its users wherein every behavior takes in the minimal number/type of inputs and gives the maximum relevant output defined by the context."

Any technical system can be defined from a user's perspective using the above definition.

Hemant Jha
Founder - VPlanSolutions
Researcher, Trainer