Provide Effective Feedback and Guidance and Assistance 595

Step 9: Provide Effective Feedback and Guidance and Assistance 595

For data entry tasks, research has found no advantages for having response times less than 1 second.

If human task closures exist, high levels of concentration are not necessary, and moderate short-term memory requirements are imposed; response times of 2 to 4 seconds are acceptable. If major task closures exist, minimal short-term mem- ory requirements are imposed, and responses within 4 to 15 seconds are accept- able. As the response-time interval increases beyond 10 to 15 seconds, however, continuity of thought becomes increasingly difficult to maintain. Research in applied attention and cognition suggests that after 10 seconds or more of wait time, the boundaries of the current “unit task” are broken (Card et al., 1983). It has been postulated that with long delays the sequence of actions stored in short- term memory become badly disrupted and must be reloaded.

The aforementioned response time guidelines, then, relate to the general tasks being performed. Their applicability to every situation can never be guaranteed.

Satisfaction with response time is a function of expectations. Expectations are based, in part, on past experiences. These experiences may be derived from working with a computer, from the world in general, or from the perceived com- plexity of the task the computer is performing. These expectations vary enor- mously across individuals and tasks.

MAXIM Service = Perception – Expectation (D. Meister, 1985). Dissatisfaction with response time is a function of one’s uncertainty about delay .

The degree of frustration with delay may depend on such psychological factors as a person’s uncertainty concerning how long the delay will be, the extent to which the actual delay contradicts those expectations, and what the person thinks is causing the delay. Such uncertainty concerning how long a wait there will be for a computer’s response may in some cases be a greater source of annoy- ance and frustration than the delay itself.

People will change work habits to conform to response time. As response time increases, so does think time. People also work more carefully with longer response times because the time penalty for each error made increases. In some cases, more errors have been found with very short response times. This may not necessarily be bad if the errors are the result of trial-and-error learning that is enhanced by very fast response times.

Constant delays are preferable to variable delays. It is the variability of delays, not their length, that most frequently distresses people. From a consistency stand- point, a good rule of thumb is that response-time deviations should never exceed half the mean response time. For example, if the mean response time is 4 seconds,

a 2-second deviation is permissible. Variations should range between 3 to 5 sec- onds. Variation should never exceed 20 percent, however, because lower response time variability has been found to yield better performance, but small variations may be tolerated.

More experienced people prefer shorter response times. People work faster as

596 Part 2: The User Interface Design Process

be useful to let people set their own pace of interaction. He also suggests that in the absence of cost or technical feasibility constraints, people will eventually force response time to well under 1 second. In general, the longer people interact with a system, the less delay they will tolerate.

Very fast or slow response times can lead to symptoms of stress. There is a point at which a person can be overwhelmed by information presented more quickly than it can be comprehended. There is also some evidence indicating that when a system responds too quickly, there is subconscious pressure on people to respond quickly also, possibly threatening their overall comfort, increasing their blood pressure, or causing them to exhibit other signs of anxious behavior. Symptoms of job burnout have been reported after substantial reductions in response time.

Slow and variable response times have also been shown to lead to a signifi- cant build-up of mood disturbances and somatic discomfort over time, culminat- ing in symptoms of work stress, including frustration, impatience, and irritation.