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Saturday, October 22, 2005

The Miller Response-Time Test

In the field of Human-Computer Interaction, HCI for short, one crucial and much-studied aspect is the speed of the computer's response to various kinds of user inputs. Although a few of these studies do get quoted in discussions of Web site design and Web usability, most books and articles on these topics devote very little space to this aspect.

One of the best summaries appears in Andrew B. King's book Speed up Your Site, which opens with the simple observation that People hate to wait. His first chapter (Response Time: Eight Seconds, Plus or Minus Two) includes a brief history of HCI research into the influence of computer response time on user satisfaction (or frustration), and an overview of its relevance to Web site usability.

In the early days of the Web, Service Level Management meant little more than keeping your site up and running. Then came the 8-second rule of Web page download time, as people began focusing on the need to build and maintain a consistently responsive Web presence. This rule originated in research results presented by Peter Bickford in his paper Worth the Wait?, published by Netscape in 1997.

For a while, Bickford’s paper was widely quoted whenever Web site performance was discussed, and the 8-second statistic soon took on a life of its own as a universal rule of Web site design. Bickford’s actual results however were more variable that this simple “rule” might suggest. Only half the test subjects abandoned after 8.5 seconds, and site feedback (like animated cursors or progress bars) kept them around for much longer.

Interestingly, the 8-second rule was by no means the first widely accepted rule in the field of human computer interface design:

  • In the world of hypertext systems (which predate the Web and HTML by about 25 years), Akscyn’s Law was well established prior to the first Hypertext Conference in 1987. It states: “Hypertext systems should take about 1/4 second to move from one place to another. If the delay is longer, people may be distracted; if the delay is much longer, people will stop using the system”.

  • As long ago as 1968, when all computers were mainframes, Robert B. Miller’s classic paper on Response Time in Man-Computer Conversational Transactions described three threshold levels of human attention. A response time of one tenth of a second is viewed as instantaneous, a response within 1 second is fast enough for users to feel they are interacting freely with the information, and response times must stay below 10 seconds to keep the user’s attention focused on the dialog (note that this is similar to Bickford’s findings). Miller also concluded that a consistent 2 second response would be ideal.

Many researchers have investigated the subject of Web responsiveness since Bickford, but no new universal rules have emerged. This is no surprise, because Miller’s findings were a direct result of how people’s brains operate, and they applied to any human interactions with machines. Changing the machine from a mainframe terminal to a Web site has not changed people’s brains.

So how does the Web experience today measure up to Miller's norms? Jakob Nielsen has been writing about this aspect of Web usability for over 10 years, and although he is still pessimistic about Internet technology in general (see The Need for Speed) the very best and most popular sites are actually doing OK these days. The home page download times of the top 10 or so sites in The Keynote Business 40 index do achieve Miller’s 1-second threshold (for users with a high-speed connection). Many more pages achieve Miller’s 2-second guideline.

But it’s still safe to say that the vast majority does not yet pass the Miller Response-Time Test of computer usability. So if your site's pages are slower, then you still have some work to do before you can feel assured that their design is ideally suited for human-computer interaction. Dogs maybe, but not humans.


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