A user-friendly interface is paramount if you want to keep a user engaged with your app. But what, precisely, makes an app user-friendly? That's a complicated science, with detailed books and research papers devoted to it. We thought we'd go down an unconventional route and instead of asking people who design apps for sale, we thought we'd ask those who design and analyse apps for internal use at big corporations, where efficiency is paramount.

All agree that if your app has any of the following three elements, that's probably not a good sign.

1. Too many places to make choices, especially on the same screen

"If you're ever working with a screen that has 40 input dialogs and drop-down menus and radio boxes, it's not really because you have to do 40 different things," says Brian Fino, managing director of Fino Consulting, which advises companies on IT investment.

"If you're designing a simple, well-thought-out interface with just a couple of discrete controls, that's easier to use and also easier to test. So hopefully it will be a more stable app."

2. Too many screens

"There are application development principles that really transcend the question of enterprise or consumer," says Gartner analyst Bill Clark. "One of them is the 'three-click rule.' Any time a population has to click on a link or traverse through to another screen, you lose about half of them, in terms of their paying attention or keeping the context of what they were doing in their heads. So after three clicks, you've lost most people."

3. Too much functionality

Consider the first two items in combination. If you can't bunch too much on one screen, and you can't have too many screens, then you're led to an inevitable conclusion: You just can't have too much stuff.

And that's a good approach, according to Mike Croucher, head of IT architecture and delivery at British Airways. "You can't make these things too complex," he says. "You've got to really think about the amount of data and options you can ask people to select in any one transaction. If you start taking people through too many transactions, they start losing the will to live. So you have to look at the value of each piece of information you put on the screen."

The solution, he says, is not to try to create an app that will solve all problems for all users. "Our applications tend to be delivered on the 80/20 rule. That is: Deliver something that does 80% or 90% of what you need well, and ignore the rest so that you don't overcomplicate it."