Accidental manipulation and intentional affordance

In my post Getting started in interaction design, I recommend that you pay attention to design and learn to appreciate good design. I had a chance to do both last weekend. I was at Stowe Mountain Lodge, my favorite resort in Vermont, and noticed that the controls on their Technogym exercise equipment were particularly well designed. To appreciate why, I’d like to start by explaining a significant problem with direct manipulation, and give a couple examples of direct manipulations that aren’t so well designed.

Direct manipulation

Direct manipulation is a crucial ingredient of a modern UX. Direct manipulation is the ability for users to perform a task on an object instead of indirectly through some intermediary. For example, moving a window using a Move Window menu command is indirect manipulation, whereas dragging a window into position is direct. Traditionally we have used a mouse to perform manipulation, but touch-based interfaces feel more direct because there’s nothing between users and the object being manipulated.

Accidental manipulation

But there’s a problem: where there is direct manipulation, there is accidental manipulation. There’s always a chance that the user performed an action either entirely by accident, or performed the wrong operation or acted on the wrong object.  The potential for accidental manipulation is large enough that critical operations shouldn’t be performed without confirmation. (Hmm…did I just drag the North American domain to Fiji?) But note that generally, combining direct manipulations with confirmations is a poor idea because confirmations destroy the very directness you are after.

Touch-based UIs are especially prone to accidental manipulation. This is why a swipe motion (with very little margin for error) is used to initiate interaction with an iPhone—a correctly performed swipe indicates with high probability that the input is intentional.

Well designed direct manipulation must minimize the likelihood of accidental manipulation. Failing to do so will make the design annoying at best and disaster prone at worst.

Bad example: My Miele vacuum

My Miele vacuum is an example of a design that fails to consider accidental manipulation.

Take a look at the handle, and notice the bypass device at the top. Not obvious from the photo is that the bypass device slides open with minimal resistance. Where do you think your thumb will naturally go? When vacuuming, what motion will you naturally make? Guess what happens—constantly—when you naturally use the device as intended.

This vacuum is frustrating to use because I’m constantly opening the bypass without realizing it. The feedback isn’t as obvious as you might think—the vacuum doesn’t work well and sounds louder and eventually you realize that you’ve opened the bypass and wonder how you managed to open it yet again.

Talk about not getting the proper amount of suction.

Another example: Windows 7 Aero Snap

To take a software example, this extremely heavily promoted feature allows you to quickly view two windows side by side by dragging them to their respective sides of the screen. (Snarky comment: from the advertising, you’d swear that this was the most useful thing you could do with a computer.)

Unfortunately, this feature wasn’t designed to prevent accidental manipulation. If you don’t want a window to snap, it’s very difficult to persuade Windows otherwise once you get in Snap’s gravitational field. (“Aero Shake” cancels a snap, but shaking isn’t easy to do and it’s difficult to discover.)

Aero Snap works extremely well better with high resolution, wide-screen monitors, but I find that it’s way too easy to trigger by accident with lower res monitors (like 1024×768). In this case, the accidental snaps happen far more frequently than intentional ones, making the feature a net negative.

This part of Windows 7 wasn’t my idea.

Update (5/11/2011): I finally got fed up and turned off Aero Snap. I find that even on high res monitors, it’s wrong far more often than it’s right. The novelty has long worn off, so it’s just not worth the trouble anymore.

Great example: Technogym

I found that the Technogym level controls are brilliantly designed to prevent accidental manipulation. Here is what the control looks like:

Note how recessed the button is. Here’s how the handle looks when you grab it naturally:

Note how the control lies in the gap between the thumb and the fingers, and how the thumb is naturally away from the control. However, even if you were to slide your thumb around in a variety of positions, you won’t trigger the control accidentally because it is recessed:

But if you do want to change the level, doing so is simple and easy.

The goal is to make it difficult to press the button through casual contact. If you press the button, you almost certainly did so deliberately.

Intentional affordance

I use “intentional affordance” to describe direct manipulation that ensures the interaction is intentional. The challenge is to design simple, natural interactions that are constrained just enough to signal clear intent. The Technogym controls and the iPhone swipe are excellent examples.

If you do only one thing:
Design manipulations that are likely to be performed accidentally to have affordances that require clear, deliberate intent.

The tragedy of great design

The tragedy of great design is that if it is particularly well done, most people won’t even notice it. Their reaction will be “Of course! How could you design it any other way?” I bet most people who used the Technogym controls never gave them a second thought. But the bad examples show that great interaction design happens only when someone designs something carefully and well.

Fixing Aero Snap

As a challenge, design Aero Snap to have intentional affordance. When done, compare your answer to mine.


Leave a Comment / What do you think?

3 Responses

  • Ned Smith - May 15, 2010 at 10:41 pm #
  • Hi Everett,

    Giving Aero Snap Intentional Affordance challenge: My first sketch was to require the user to have to move the window farther to the side. I was excited to see your answer on that. This was a great article.

    Very timely considering direct manipulation could be a new frontier for devs creating apps for the Windows Phone environment. I’m looking forward to it.


  • Luca Candela - June 11, 2010 at 6:32 am #
  • I don’t have the slightest hint of a problem with Aerosnap. Never triggered the feature accidentally. Are you looking for something wrong in Windows 7 for the sake of writing a blog post, or do you really have that much of a problem with it?


  • Everett McKay - June 11, 2010 at 8:53 am #
  • As I say in the post, Aero Snap works extremely well with high res, wide screen monitors, but I find that it triggers accidentally quite often with low res monitors (1024×768). That’s the nature of most accidental manipulation–the design works well when used exactly as intended, but the designers fail to recognize that there are other situations.

    There are a couple important design principles here. First, as Don Norman points out in The Design of Everyday Things, is that when people make mistakes they tend to blame themselves instead of the design. If you click something by accident, you think “Oh, I shouldn’t have done that” instead of “Hmm…could this design be improved in order to prevent that accident?” Once you train yourself to think this way, you being to see ways of improving designs that you otherwise would have missed. Interestingly, even if Aero Snap had tested extensively in 1024×768, it’s possible that nobody would have recognized this as a design problem.

    Second, users don’t understand exactly how things are supposed to work and therefore make small mistakes all the time. Great design accommodates those mistakes. This is an “attention to detail” area that we often neglect. For example, in Windows sometimes you single click to make things happen, other times you double click. Users don’t always know which action to take, so sometimes they double click when they should single click. Try doing that on a notification area icon. What you’ll find is that single click displays/hides the icon’s UI, but double clicking flashes it. It’s never the user’s intention to flash the UI. A better design would assign the single click behavior to double click. That would eliminate the accident, and there’s no downside to doing so.


For more information, please contact info@uxdesignedge.com

All Content Copyright © UX Design Edge