When it comes down to it, interface design is a simple equation:
X + Y = Z
X is the user, Y is the system, and Z is the result:
User + System = Result
What I find interesting about this equation is that the output of your work as an interface designer is not the user, or the system, or the result. What you work on directly is the plus sign, which is the interface between user and system.
Like a plus sign, your interface needs to be additive. It needs to take the user and the system and make sure that the result is more than either of them alone.
If you’re not careful, the plus could become a minus, or even worse a division. The result could become less than what you started with. For example your interface might be so difficult to use that people do less work than if they’d done it manually.
If you’re good, or lucky, the plus sign could instead be a multiplication, which means your result is even better. Maybe your interface makes possible something that no person could ever have dreamed of, even if they worked at it manually for hours on end.
As a designer you’ll likely come across situations where different parts of the equation are unknown.
You could have a system already, and know what result you can achieve, but don’t know what type of person would benefit most: X + 3 = 7. This is common when you want to expand into a new market and need to do research to learn which is most suitable.
Maybe after a lot of research you understand the types of people there are and the results they want to achieve. Now you need to build the system and design the interface: 4 + Y = 7.
Or maybe you’ve stumbled across a problem and you need to know if other people have the same problem, and whether it’s possible to solve this problem with technology? That’s X + Y = 7.
However you approach the equation, if you don’t understand each part of it you’re going to run into issues.