CMSC 434/828S - Fall 2003
Prof Guimbretiere

Introduction to Human-Computer Interaction
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Homework #3:
(Due 10/14/03)

1) (30 points) Fitt's law

(10 points) Use Fitt's law to compute the time it will take to key in the following key sequence (starting with the finger on top of the "ON" key):

1, 2, "ENTER" key (on top of the first E), 8, +, 2, ÷, "Öx". (HPs use the Inverse Polish Notation. The sequence represents SQRT((12+8)/2)).

You might assume IM = 100ms, each key is .3"x.3", row are .44" apart center to center, column are .44" apart center to center and  a 200ms dwell time (delay) between keys. Explain your answer.

(10 points) The manufacturer of this device decides to increase the device dimensions by 25% (uniform scaling of all parts). How will this influence the time it takes to perform the sequence above? Explain your answer.

(10 points) The manufacturer decides to increase the distance between each key by 25% (in both directions), but to save money use the exact same key size as in the original device. How will this influence the time it takes to perform the sequence above? Explain your answer. 

2) (30 points) The Human Information Processor

(10 points) Using the following figure, name and describe briefly each element of the Human Information Processor.

(20 points) In an experiment, subjects are presented with 2 symbols in sequence and instructed to press the “Y” key only if the two symbols are identical. In one trial, a symbol is presented on the screen, and then the same symbol is presented again. Describe how the different elements of the Human Information Processor interact up to the point where the key is pressed by the subject (no computations is required).

 

 3) (15 points) Skill acquisition

Name and describe the 3 stages of skill acquisition. Illustrate each stage using "learning how to swim" as a running example.

4) (10 points) Echo

You are standing in front of a large concrete wall. Behind, you a friend is blowing a horn creating short pulse of sound every 5 seconds. How far from the wall shall you be to perceive an echo? Explain your answer. (you might assume that the speed of sound is 330 m/s).

5) (15 points) Memory

For each of the following scenarios, indicate whether short term or long term memory is used. Explain all your answers.

  • Looking up a phone number in a phone book before dialing it

  • Remembering the birthday of your best friend

  • Remembering a keyboard shortcut

  • Answering the last question in an instant messaging session

  • After looking in the online help, perform the sequence of operations described there