A Foolish Consistency in User Interfaces

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Designers evangelize for consistency. Consistent interfaces are easier to understand and generally provide smoother, more pleasing experiences. Consistency is an easy idea for organizations to grasp, and the remedy for inconsistency is clear: Look for all the things that aren't the same but should be and make them the same. It's also easy to become so attuned to insignificant differences that we insist on consistency for its own sake. Like Emerson, I'm arguing today against "a foolish consistency" and in favor of purposeful inconsistency.

The key word in that last sentence is purposeful, because haphazard inconsistency is clearly a detriment to a positive user experience. Why? Change is salient. Human beings are hardwired to notice when things in their environment change. Paying attention to change was a good survival strategy and is largely responsible for our ancestors not getting eaten by tigers that they failed to notice. For design, the implication is that when something in the interface changes, users notice.

Generally, having users' attention is a good thing, as I explained in the column "May I Have Your Attention Please?" (March/April 2012). But when a user's attention is captured by an irrelevant change, it interferes with the user completing his primary task. I experienced this the other day while using the automated checkout lane at my local grocery store. The store had updated the wording of some messages, which should have been a good thing for user experience. But because the new messages were recorded in a different voice, I found myself thinking, "Hey, who's the new guy?" and forgetting to scan the carton of milk in my hand. It's not that I particularly cared about the new voice; it was just impossible not to notice it.

Later I remembered a discussion I had with one of my clients that had taken the consistency lesson to heart and become expert in pointing out inconsistencies in ongoing design projects. While reviewing design documentation, the client noticed that one path included an acknowledgment prompt after the user speaks and another path did not, and they asked me to standardize this. I argued against this because I realized that most users would never be in a position to observe this inconsistency and even if they did, there were no negative consequences to it. The only reason my client noticed the inconsistency is that we were looking at all possible paths through the system at once. Of course, there is value in reviewing interactions globally, from the outside, but we also need to remember that this is not the way that users experience the system. As long as users can achieve their goals, they're unlikely to notice, care about, or remember inconsistencies among different parts of the experience.

So when is consistency necessary and inconsistency acceptable?

Consistency matters:

  • In the presentation layer of the interface, which for speech means the voice and audio quality of prompts. Take a lesson from visual designers, who often use color change to capture the user's attention, and remember that users can't help but notice when the interface suddenly sounds different.
  • In the tone of the conversation. People are finely attuned to nuances of conversation, so they notice when wording changes from casual to formal, or when they're no longer allowed to interrupt. Consistency in the overall tone of the conversation puts users at ease and allows them to focus on their goals.
  • In rules of engagement, that is, in the kinds of spoken responses they're expected to give. Users are becoming more accustomed to giving free-form responses in automated systems, but most systems still require more structured directed dialogue type responses as well. The transition between saying what you want and speaking one of a few specific options is noticeable and tricky for many users.

Consistency doesn't matter:

  • For modality changes. Users happily switch between speaking, tapping, and swiping and between reading and listening. Don't force an interaction to stay within a modality for the sake of consistency alone.
  • For synonymous wording. Users understand that we're talking about the same thing if an application says order status in one place and the status of your order elsewhere. The same feature should be referred to consistently within an application, but needn't be rephrased to fit the specific situation.
  • For handling across different paths. There are always multiple paths through an automated system, and it often makes sense to design different interactions for these situations. Users aren't concerned about getting identical treatment in different situations—they care about completing their tasks. The guiding principle should be to design the most effective interactions for each situation, not to treat each option identically.
    Susan Hura, Ph.D., is a principal and founder of SpeechUsability, a VUI design consulting firm. She can be reached at susan@speechusability.com.

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