Powers of 10: Time Scales in User Experience

The user experience field has its own version of "Powers of Ten" (the classic 1968 documentary by Ray and Charles Eames). For us, it's not so much that things get 10 times bigger or smaller; most user interfaces are about the same physical size , as dictated by the need to work with the human body. For example, a BlackBerry keyboard is about 1/5 the size of a PC keyboard — it wouldn't work at 1/10 the size. And, except for wall-sized displays, nothing is 10 times bigger than a PC interface.

But in the "4th dimension" of time , user experience phenomena work across many powers of 10.

0.1 Second

Many of the effects of perceptual psychology take place at this time scale.

A research team lead by Dr. Gitte Lindgaard found that people can make rough decisions about a Web page's visual appeal after being exposed to it for as little as 50 ms , which is 1/20 of a second (50 ms is only half of 0.1 second, but it's close enough for the purposes of a "powers of 10" analysis.)

In Lindgaard's study, screen images were flashed at test participants for 0.05 seconds, after which they could distinguish between more and less attractive designs. It's important to realize that this is not how users actually approach Web pages during real use. For one, pages don't flash on the screen for an instant and then go away. Instead, they render over a period of a second (if we're lucky — otherwise more). Second, people spend a few seconds looking over the page before they decide what to do about it.

Still, the study does show that people can form basic visual impressions very quickly, at the limits of human perception.

0.1 second is the response time limit if you want users to feel like their actions are directly causing something to happen on the screen . For example, if you click on an expandable menu and see the expanded version in less than 0.1 seconds, then it feels as if you made the menu open up. If it takes longer than 0.1 seconds for the revised state to appear, then the response doesn't feel "instantaneous" — instead, it feels as if the computer is doing something to make the menu open.

Thus, to create the illusion of direct manipulation , a user interface must be faster than 0.1 second.

In eyetracking studies , most of the fixations we track last little more than 0.1 seconds. In fact, the first thing people notice when running their first eyetracking study is how fast the human eye moves across Web pages (or other stimuli). Users look at things very briefly, which is a big reason to emphasize clarity in content usability .

1 Second

When the computer takes more than 0.1 second but less than 1 second to respond to your input, it feels like the computer is causing the result to appear. Although users notice the short delay, they stay focused on their current train of thought during the one-second interval.

This means that during 1-second response times, users retain the feeling of being in control of the interaction even though they notice that it's a 2-way interaction (between them and the computer). By contrast, with 0.1 second response times, users simply feel like they're doing something themselves.

For Web usability, this means that new pages must display within 1 second for users to feel like they're navigating freely ; any slower and they feel held back by the computer and don't click as readily.

In the Web's early days, it was impossible to achieve such download times. This is why many guidelines advised that you minimize the number of page views required: back then, going to a new page was unpleasant if it took more than a second to do so.

Today, with broadband widely available, subsecond download times are eminently possible and should definitely be the goal. The main problem now is not so much big graphics or heavy "page weight" (number of kilobytes to download). Now, slow response times are more frequently caused by excessive widgets and other dynamic elements that bloat the design and slow down the server.

(Also, it's important to remember that some people still use dial-up, especially in rural areas or developing countries. Mobile devices also have slower connections, so your website's mobile version usually needs a serious diet.)

10 Seconds

After 1 second, users get impatient and notice that they're waiting for a slow computer to respond. The longer the wait, the more this impatience grows; after about 10 seconds, the average attention span is maxed out. At that point, the user's mind starts wandering and doesn't retain enough information in short-term memory to easily resume the interaction once the computer finally loads the next screen.

More than 10 seconds, and you break the flow . Users will often leave the site rather than trying to regain the groove once they've started thinking about other things.

10 seconds is also the time users typically allocate to examining a page before deciding that it's so bad that they're going to leave.

The average page visit lasts about 30 seconds , but the more experienced the users are, the less time they allocate to each Web page. People are impatient on the Internet. Instantly gratify them, or they're out.

1 Minute

Users should be able to complete simple tasks in about 1 minute. Awkward sites that require much more than a minute for basic tasks — such as transferring money from a savings to a checking account — will be abandoned.

Likewise, most Internet videos should last no more than 1–2 minutes because people don't like passively watching something for much longer than that while they're in the active frame of mind induced by Web surfing.

Most website visits last about 2–4 minutes.

10 Minutes

10 minutes would be a long visit to a website. In one case, for example, we followed a user researching a B2B purchase across 25 site visits . The longest site visit? 7 minutes.

1 Hour

Most usability studies last from 1 to 2 hours because it's hard to recruit users to come in for longer tests. In fact, unless we're testing kids — for whom an hour is the max — we usually limit our test sessions to 90 minutes. People tire after an hour or two.

People complete most Web tasks in less than an hour. In one study, 1/2 of e-commerce purchases occurred within 28 minutes of the user's arrival at the website. Of course, the other half were spread across longer intervals, often including multiple visits that were days apart.

1 Day

1 day is the maximum turnaround for customer service requests, although you should send transactional email and confirmation messages within 1 minute to keep users from wondering whether their action — such as a purchase or address change — has been received correctly.

The difference here is that users assume that customer service requires human intervention, so they don't think the computer is broken if they don't hear back within 1 minute. Faster service is still appreciated, of course.

Many users habitually check certain content sources on a daily basis. (A behavior called monitoring.) So, if your topic warrants, it might be smart to publish a daily e-mail newsletter . (But be warned: you're begging for unsubscribes if you have a slower-moving topic and publish too often.)

1 Week

Other habits are weekly (or monthly, or yearly — depending on seasons, holidays, or tax filings). In our studies of how people use social networking , we found that Facebook and Twitter tend to be daily habits (or more for some people), whereas MySpace and LinkedIn tend to be checked weekly.

Tasks that require extensive research or big decisions often stretch across a week or more, as users gradually progress in their thinking. So, while each individual visit to any given website might last only a few minutes, the full process takes much longer.

This means that sites must support revisitation behaviors, for example by keeping track of what users have done in previous visits.

Weekly and monthly behaviors don't lend themselves to in-lab usability testing, so we often study them through diary studies or other more field-oriented methods.

1 Month

Business processes often take even more time than individual decisions because of the need to get various people on the same page. For both B2B sites and enterprise collaboration , it's common to have a month or more pass between the initial action and the completion of that workflow.

1 Year

Once people have used a website for about a year, they graduate to being experienced users with some knowledge about how the site works. It takes a long time to build such expertise because of the superficial in-and-out way users approach websites. Each visit is short, and people don't spend much effort on explicitly seeking out new features and building their skills.

Nevertheless, people do eventually learn something about the sites they use frequently. This is why Amazon.com can get away with very complex product pages that are cluttered with more features than I recommend for most sites: many of Amazon's customers have used the site for years and thus have the background required to cope with the site's numerous features.

Organizational change usually takes years. For example, it typically takes 2-3 years for a company to progress to the next level of the 8 stages of usability maturity .

10 Years

It takes almost 10 years for users to develop deep expertise in a complex system such as Unix. The learning curve thus continues for many years past the initial uptake of simple features that we usually study. Throughout these first 10 years, people gradually explore more and more corners of the system and slowly build their skills.

Data often lives for decades — far longer than any individual user interface that people use to access the data. This means that you need migration tools to help users make sense of old data and transfer it into new systems. For example, photo sharing sites should give users tools to import old photos. Eventually, such sites will also need to cope with the exploding mass of thousands of decade-old uploads.

100 Years

If organizational change takes years, social change can take decades, getting close to the 100-year mark in some cases, which is why we're not seeing them yet for many aspects of computing. For example, it's possible that collaboration systems will depopulate cities . It's also possible that the change to shorter, more superficial information nuggets instead of immersive, linear information will undermine education as people lose the ability to learn and study harder concepts.

We don't yet have 100-year effects in user interfaces, because people haven't yet used computers for their entire lifetime. But eventually this will happen. We're already 20 years into the era where many people started using computers as little kids. And we're certainly seeing more and more uptake of Web use and other computer activities among senior citizens .

Combine these two trends, and we'll eventually have people who've used computers all of their lives — or 100 years for the more long-lived members of the species. What'll that mean? Let's discuss this in 80 years.