Summary: The more engaged users are, the more features an application can sustain. But most users have low commitment -- especially to websites, which must focus on simplicity, rather than features.

In designing any user interface, one of your key decisions concerns the tradeoff between features and simplicity. The more features, the more complicated the system inevitably becomes:

Features have to be shown to users, so screens get busier.

to users, so screens get busier. Menus get bigger and/or more numerous, making it harder for users to find the features they need.

the features they need. Features must be explained , ballooning the size of the help system and/or the manual: Fatter documentation takes longer to read and makes it harder for users to extract a good conceptual model of the system. More docs also make it harder for users to find the explanations they need.

, ballooning the size of the help system and/or the manual: Each extra feature offers more rope for users to hang themselves: they're more likely to use the wrong feature , either as an error of intent (a mistake caused when they think the wrong feature is the one they need) or as an error of execution (that is, a slip, as when they click the wrong button in a crowded toolbar). Conversely, Steve Jobs famously defended the Mac's one-button mouse by pointing out that users would never click the wrong mouse button.

, either as an error of intent (a mistake caused when they think the wrong feature is the one they need) or as an error of execution (that is, a slip, as when they click the wrong button in a crowded toolbar). Conversely, Steve Jobs famously defended the Mac's one-button mouse by pointing out that users would never click the wrong mouse button. The number of feature interactions grows by the square of the number of features: more can go wrong, and it becomes harder for users to understand why a change in one corner of the system has an effect in another corner.

grows by the square of the number of features: more can go wrong, and it becomes harder for users to understand why a change in one corner of the system has an effect in another corner. The more options users have to choose from, the more time it takes their brains to prepare for action and decide what to do. Even if a fancy feature can theoretically execute a task faster, overall system use often slows because users spend more time on the mental operations required to choose from among features than they save from the more efficient feature.

The answer seems clear: minimize features and chase simplicity at any cost. This is indeed the case for most user interface design, but not for all projects.

Right-Click: An Extra Feature That Works

Mouse buttons are a great example of a case in which the benefit from additional features is worth more than the penalties outlined above. Academic studies have found that common GUI operations are substantially faster with a two-button mouse than with either a one- or three-button mouse. And the most commercially successful GUI does indeed use a two-button mouse.

Two weeks ago, I observed dozens of average-skilled business users as they attempted common business tasks with two high-end applications. Even though these people were neither geeks nor experts in the software we tested, most of them frequently used right-click shortcuts. The contextual pop-up menu was often their operation of choice, allowing for great efficiency in many tasks.

(Before you redesign your user interface around right-click pop-ups, be warned: less skilled users rarely use these menus.)

Right-click helps medium-skilled users because it's a consistent interaction technique that works the same everywhere. (Indeed, high-skilled users are often disappointed when an application doesn't support right-click — for example, if it's implemented in Flash and brings up the Flash player menu instead of contextually-appropriate application commands.)

Right-click also works because business professionals and other mid-level users typically depend on their PCs and are willing to learn a few techniques to use it better.

User Engagement Levels

Users' willingness to learn is the most important factor in how much complexity you can allow in the user experience. If people are extremely excited about a user interface, they'll welcome more features and will spend the time to figure them out.

Mostly, though, users have a low engagement level with user interfaces and just want them to get out of the way. People don't want to spend time learning , they want to spend time doing — a well-documented effect called the paradox of the active user. (It's a paradox because people might save time in the long run if they spent more time learning about powerful features. But, empirically, users almost never want to do this, and you should design for how people actually behave, not how you wish they behaved.)

An example of a high-engagement design is the seventh and final book in the Harry Potter series. The first 100 pages of Harry Potter and the Deathly Hallows make no sense unless you're intimately familiar with the first 6 volumes in the series and can remember every detail about a profusion of magical objects and earlier plot twists. I found it a bit difficult to follow the story in Potter #7, but on balance, I think the author made the correct design decision in targeting customers who were very strongly engaged in the series: People aren't likely to buy #7 if they haven't read the earlier books, and Harry Potter fans tend to be on the fanatical side of fandom.

Traditionally, the design of a book series caters to lower-engagement readers by adding exposition that brings them up to speed. But — at 759 pages — The Deathly Hallows is already fat enough that adding, say, 50 pages of background material would have pushed it over the edge and made it less enjoyable for most readers.

3 Levels of Photoshop Complexity

Adobe ships three Photoshop versions targeted at three different user engagement levels:

Photoshop CS (list price $650), targeted at professional graphics artists and photographers. Photoshop Elements (list price $100), targeted at photography enthusiasts and people who want to do basic image manipulation, such as cropping screenshots. Photoshop Album Starter Edition (free), targeted at average consumers who've just bought a digital camera and might want to fix a redeye or brighten a dark image.

The professional version is not only expensive, it comes with a manual that's several hundred pages thick and has an entire ecosystem of training seminars and optional books (B&N sells 1,622 books with "Photoshop" in the title). Photoshop CS is so complicated that to learn to achieve certain effects, you have to spend a full day with a Photoshop guru.

However, the very success of such training products proves that users will gladly pay more than the price of the software to learn how to use it. For professional users, Photoshop creations are their work products, and being able to make a picture look better is worth almost any amount of training and user interface complexity.

To its credit, Adobe recognized that Photoshop CS is much too complicated for people who just want to clean up their personal photos. Most users don't need that many powerful features, and they certainly don't want to read beefy manuals and extra books just to get a better-looking vacation snapshot.

The Photoshop Elements manual is both thinner and easier to read than the CS manual. And the documentation for Photoshop Album is only 20 pages long.

Each of the three versions is appropriately targeted at a particular level of user engagement, from people who care passionately about image manipulation to those who aren't particularly interested in graphics software.

Shallow Engagement with Websites

Where does your website fall on the 1–3 scale of user engagement we saw for Photoshop? Outside the scale, at level 4. People don't want to read 20 pages of instructions to use a website. They demand instant gratification or they leave.

The user engagement level with websites is incredibly low, as dictated by information foraging: people don't commit easily to any individual site, because it's so easy to get to other sites. Skimming the cream from each site is usually the superior browsing strategy.

As studies in my recent book document, users visiting a new site spend an average of 30 seconds on the homepage and less than 2 minutes on the entire site before deciding to abandon it. (They spend a bit more time if they decide to stay on a site, but still only 4 minutes on average.)

For an example of user fickleness and lack of engagement with each individual site, see the transcript of a B2B user session.

Different engagement levels are one of the main reasons we have a one set of usability guidelines for websites and a different set of guidelines for applications.

Intranets typically sustain mid-level user engagement because a company usually has only one intranet. That's one key reason that we have hundreds of separate intranet design guidelines. (The other reason is that employees' intranet tasks differ from the tasks that customers perform with websites.)

Be aware that not all applications can expect deep user engagement levels. Particularly for ephemeral applications embedded within websites (say, the configurator on a car site), users often have close to zero commitment: if an applet's purpose isn't immediately obvious or if features are too complex, people will leave a Web-based application just as readily as they'll leave a content page.

When we tested the investor relations sections of corporate websites, we found that most individual investors were confused by advanced charting tools or other features that went beyond very simple interaction techniques. Even though investors have thousands of dollars at stake, they still prefer websites to be simple because they have to deal with multiple sites.

Loyal Users: Higher Engagement Levels?

Some websites can build user loyalty and grow user engagement levels across subsequent site visits. Such sites can gradually introduce more advanced features for those users who become sufficiently committed to the site.

A famous example is Amazon.com's one-click shopping feature. It's complicated to understand, and pretty scary to boot. Still, one-click shopping helps some users. Amazon's usability rules differ from those for most other sites because it's big enough and established enough that many users have established a close relationship with the site and are thus willing to engage at some level of depth.

But even Amazon makes it easy to shop in the traditional way by using the shopping cart, which is a design pattern that everybody knows by now. Indeed, I strongly advise the vast majority of websites to scale back their features and dramatically simplify the user experience for initial use . After all, to progress to the deeper engagement levels, prospective customers must first successfully pass through the initial use phase.

Typically, when new prospects first visit your site, you're simply one of ten sites on the SERP. The only way they'll shortlist your site is if you can convince them in two minutes.

Thus, websites should have almost no features: focus on the words.

To determine how much complexity you can afford in a user interface, you must analyze user engagement levels: Do they care deeply, or do they just want to get something done as quickly as possible? Typically, users care less than you think! You're not important to them. This is one of the main reasons companies need systematic usability studies: to make explicit the fact that outside customers don't find your design as important as you do (because you work on it all year).