Response Time and Display Rates

Response Time

• Number of seconds from key press till computer display signals task completed
• Delays longer than 15 seconds cause users problems
• Modest variations in response time (+50% or -50%) are tolerable to most users
• Most users expect 2 second delays

Display Rates

• Fast display rates cause user errors for tasks which require scanning and recognition

Theoretical Foundations

• Short-term memory (STM) capacity 7 plus or minus 2 items
• Chunking can be used to increase size of pieces
• Chunks can be retained in STM for 15 to 30 seconds
• STM is used with working memory (WM) for information processing and problem solving
• It is easy to overload STM and WM during decision making tasks

Response time can be a source of errors if long delays prevent user from implementing problem solutuion before it fades from STM or WM.

Optimum Problem Solving Conditions

• User has adequate prerequisite knowledge of task and problem environment
• Solution plan can be carried out without delay
• Distractions are eliminated
• User anxiety is low
• Feedback on solution progress is given
• Errors can be avoided or easily corrected

Some Conjectures Requiring Verification

• Novices like slower response times
• Novices prefer to work at slower speeds than knowledgable frequent users
• User like to work quickly when errors have small penalties
• Users like to work faster on familiar tasks that are easy to comprehend
• If users have experienced rapid performance in the past, they will demand it in the future

User Productivity

• On repetitive tasks, if response time is too quick users anticipate responses and make them without checking
• When monitoring a screen and making "adjustments" users do better with short response times
• For problem solving tasks, users will change task response behavior as task difficulty increases

Variability

• People want reduced variability in their lives
• Good to have similar response times for all commands in a given class
• Modest variability in response time seems to encourage people to think more
• Frustration emerges for delays that are "twice their normal time"
• Anxiety emerges if delays are 1/4 the expected time

Response Time Guidelines

• Users like shorter response times
• Long times (>15 sec) are disruptive
• User profiles change with response time
• Increasing pace increases error rates
• Response time should be matched with task difficulty
• Users should be advised of long delays
• Modest variability is acceptable
• Unexpected delays can be disruptive
• Empirical tests can help select optimal times