Jeff Offutt http://www.cs.gmu.edu/~offutt/
SWE 205
Software Usability and Design

• Some knowledge for using a UI is in our heads
– Try arranging letters on a keyboard without looking
• We use software correctly by merging our knowledge with external influences :
1. Information in the world —what we see
2. We often do not need to be precise
3. Good UIs constrain our use with syntax—legos, electronic connections, radio buttons
4. Our culture constrains us—volume and headphones
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• Who knows whose face is on a $20 bill ? $50 ?
• We do not need a map to go somewhere we are familiar with
– But we might not be able to give directions
– And what if we can’t read street signs ?
– I often forget my classroom numbers after the first week, so use information in the world to find them
• How do you remember your 50+ passwords ?
• I remember to bring my pointer to class by putting it with my notes when I prepare for class
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Head
Information that we memorize
• Our own name
• Our address
World
Information that we see or look up
• Time
• Courses offered next semester
We often move information from one place to another
• Our class schedule
• The room number for SWE 205
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1. Declarative knowledge : knowledge “ of ”
– Facts and rules : Class starts at 10:30, double click the PPT icon to open powerpoint
– Easy to write down and teach
– Usually requires memorization
2. Procedural knowledge : knowledge “ how ”
– Allows somebody to accomplish a task
– How to get to class by 10:30
– How to find the icon and double click
– Much harder to teach and learn
– Taught by demonstration and learned through practice
– Requires deeper understanding
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• People are not good at being precise
– The 1 dollar coin was almost the same size as a quarter—The US made this mistake twice in my life!
– Remembering 10-digit phone numbers is hard … so my phone has a contact list and voice activation
– The French & Indian war occurred about 20 years before the US revolutionary war … but which year?
• We learn to be precise in our field only after years of learning and practice
• A good UI must not require precision
– But should help the user find precision when required
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• Spelling is hard … but made easier by patterns
– What consonants can precede an “h” ?
– “ch”, “gh”, “ph”, “rh”, “sh”, “th”, “wh”
• Singers, musicians, dancers and martial artists don’t memorize so much as recreate based on patterns that “make sense”
• The syntax of a UI must make syntactic constraints clear
– In PPT, if I highlight text, lots of actions are immediately enabled
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This arrangement has a natural mapping … fewer mistakes !
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How can we remember which control maps to which burner ?
Mistakes are encouraged
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1. Mental : Users’ perception of reality
• Example distributed file system: files
2. Implementation : How a machine, virtual or otherwise, is actually built
• Example: Network protocols
3. Manifest : How the machine represents the implementation to the user
• Example: Local disk
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• When we push the gas pedal, the car goes faster
– Mental : Pushing makes it go faster
– Implementation : More gas to the engine, more pressure, pistons go faster, tires go faster …
• When we turn the wheel, the car turns
– Mental : Turning the wheel turns the tires
– Implementation : Turning the wheel turns something else (with the help of a motor for power steering), which causes something else to turn, which puts the tires into a different angle
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Telephones : I want to call Mom , not 1-606-XXX-XXXX
First we must put the phone number into the “world”
Compile : I want to
Run my program, not compile, link, run
File Manager : Dragging a file from window to window is :
– Move on the same disk
– Copy from USB thumb drive to disk
Calendars : Paper calendars require paging , online calendars can scroll
Interfaces should conform to the user’s mental model
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• Predicting storms
– Does the prediction “30% chance of snow” mean
• It will snow in 3 of 10 locations in the area
• Less snow than if 50% chance, but more than if 10%
• On 3 days out of 10 with these “weather conditions,” it has snowed in the past
• Dice
– If I roll a 6 five times in a row, what is the probability that my next roll will be a 6 ?
• Patriotweb
– Login-Faculty&Advisor-Summary Class List-Submit-Submit
Roster …
…
– I expect : Faculty&Advisor-Summary Class List-Submit-Submit …
Roster … but no!
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This makes perfect sense … for a paper calendar
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This is closer to the user’s mental model
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• Engineers are very comfortable thinking in terms of the implementation model
• Users are not
Adapting to user’s mental models causes cognitive dissonance
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