CMSC 198G - Spring 2008
Prof Guimbretiere

Introduction to Human-Computer Interaction

Homework #4: The Human Information Processor (100 points)
(Due 03/03/08)

1) (30 points) Fitt's law

a) (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 (finger hits the top of the first E), 8, +, 2, ÷, √x.

(This HP calculator use the Inverse Polish Notation. The sequence represents SQRT((12+8)/2)).

You might assume I_M = 100ms, each key is 0.3"x0.3", row are 0.44" apart (center to center), column are 0.44" apart center to center and  a 200ms dwell time (delay) between keys.

b) (10 points) The manufacturer of this device decides to increase the device dimensions by 25% (both directions; size and distance). How will this influence the time it takes to perform the sequence above? Explain your answer.

c) (10 points) The manufacturer decides to increase the distance between each key by 25% (both directions; distance only), 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 quantitatively and qualitatively. 

2) (20 points) The Human Information Processor

a) (5 points) Using the following figure, name and describe briefly each element of the Human Information Processor.

b) (15 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, in steps, how the different elements of the Human Information Processor interact up to (and including) the point where the key is pressed by the subject (no computations is required).

3) (10 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) Motor skill

Explain the following facts in 2-3 sentences each:

a) Even though people learn to write with their dominant hand, they can write their name with their nose or with their elbow.

b) In a driving range golfers practice aiming at targets placed at fixed distances (75 yards, 150 yards...) in front of them. It is very unlikely that they will encounter these specific distances during the game. Yet practicing on the driving range does improve one's ability to aim while playing. 

5) (15 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 should you be to perceive an echo? Explain your answer. (you might assume that the speed of sound is 330 m/s).

6) (15 points) Puzzle

You own a wolf, a goat and a cabbage and you wish to transfer them from the left bank of a river to the right bank. Your boat can hold only yourself and one piece of cargo. When left alone, the wolf will eat the goat and the goat will eat the cabbage. If you are nearby, nothing will happen.

Using the theory of problem solving presented by J. Anderson, explain how people will solve this well-known puzzle. In particular, describe the states, goals, and operators and the kind of search that might be used to solve this problem. Are all steps equally difficult to a person encountering this kind of problem for the first time? Why? Explain all your answers.