April 19, 2004“Design of User-Friendly Systems” DLC, Physical and Logical constraints Physically constrain possible operations Rely upon properties of the physical world Examples: –Large peg cannot fit into small hole –Keys can have only one of two orientations when inserted into a vertical slot Logical constraints take advantage of natural mappings and relationships Location and operation should follow these natural mappings: –Two switches to controls two lights: left switch should control left light and right switch should control right light –Last piece of a jigsaw puzzle, naturally constrained to go in the empty hole

April 19, 2004“Design of User-Friendly Systems” DLC, Cultural and Semantic constraints Cultural constraints: Knowing how to act in cultural situations such as at a restaurant Culturally acceptable behavior frames –Notice these when they are violated, such as standing in an elevator facing the rear –Differ from culture to culture Semantic constraints rely on –the meaning of the situation –the user’s knowledge of the situation and the world Example: –How to sit in a chair with arms, sit with legs over open edge

April 19, 2004“Design of User-Friendly Systems” DLC, Visibility and Feedback Visibility: “Making relevant parts [or the correct action] visible”– easier said than done Doors –Unclear design of many doors as to which side of the door opens Switches –Many switches controlling different things –Natural mapping of switches often not exploited; which direction of the switch is on/off Feedback: “Give each action an immediate and obvious effect.” Use sound to make things visible –Whistle of tea kettle when water boils –Click when toast pops up –Change in pitch when vacuum is clogged Having a good display – visual feedback –Lets the user know what effect their actions have

April 19, 2004“Design of User-Friendly Systems” DLC, Knowing what to do Constraints Physical and Logical: Good: Cultural and Semantic: Good: Bad: Visibility and Feedback Visibility: Good: Bad: Feedback: Good: Bad:

April 19, 2004“Design of User-Friendly Systems” DLC, Capture and Data-Driven Errors Capture error: When “a frequently done activity suddenly takes charge instead of (captures) the one intended.” Occurs when two different action sequences have initial stages in common, where one is well practiced and the other unfamiliar Examples: Going to change your clothes for dinner and finding yourself in bed Get in your car on Sunday to go to the store and end up going to work instead Data-driven error: Occurs when sensory data interferes with the current action sequence. Automatic actions are data-driven.

April 19, 2004“Design of User-Friendly Systems” DLC, Description and Mode Errors Description error: When the intended action has a lot in common with other actions that are possible. Comes from imprecise internal descriptions. Occurs when the wrong and right objects are physically near each other. Example: Flipping the wrong switch in a long row of switches Mode error: When controls must perform more than one action, depending on the mode Errors occur when the mode is not made visible Especially prevalent on computer systems and digital watches

April 19, 2004“Design of User-Friendly Systems” DLC, Associative activation and Loss-of-activation Errors Associative activation error: When internal thoughts and associations trigger actions. Examples: Thinking something you shouldn’t say and then saying it. Answering your telephone ‘come in’ Loss-of-activation error: Plainly, forgetting. Can happen in the middle of doing something, such as walking into a room and forgetting why you went there.

April 19, 2004“Design of User-Friendly Systems” DLC, To Err is Human Types of errors Visibility of Errors Conceptual models that lead to error Structure of tasks: Wide and deep Shallow Narrow Explaining away errors Social Pressures Forcing Functions

April 19, 2004“Design of User-Friendly Systems” DLC, Reference Norman, Donald. The Design of Everyday Things. Basic Book: New York, Chapters 4 and 5. Flanders, Vincent. “Web Pages that Suck”. ker/