Lab 4 Fitts' Law-Movement and Time

Chapter 7, Magill

Purpose:

• To examine the relationship between time and the difficulty of a task.

Fitts' Law (1954)

• Predicts the movement time for a situation requiring both speed and accuracy in which a person must move to a target as quickly and accurately as possible.
• Variables: Distance and target size
• Has been shown to generalize to a variety of physical activity and ergonomic situations where speed and accuracy play important roles.

Demonstration video:

http://www.youtube.com/watch?v=kly2QA1bFc8

Procedure:

• Marker
• Paper
• Ruler (or method of measuring 0-3 inches)
• Stop watch/ timer

Procedure:

The purpose of the lab is to assess how movement time is affected by tasks that differ in difficulty. Fitts defined task difficulty in terms of target size (W, which is the width of the target) and the distance to move (D, which is the distance between targets). He quantified task difficulty (which he called index of difficulty", of simply, ID) by using the following equation:

ID = log₂(2D/W)

For this lab eight combinations of task difficulty will be used. The easiest task has a D = 2 cm and W =2 cm. Solving the equation for ID, that would be the log (base 2) of (2X2)/2.This works out to the log₂ of 2, which is 1. The most difficult task has D = 16cm and W = 1cm. That is the log₂ of (2X16)/1, which is 5. The ID for the other task variations are listed on the lab report where the results of each trial are recorded.

Three trials will be performed on each task variation (ie - 3 trials for 8 IDs = 24 trials). The tasks should be performed in a random order (you randomize). The objective is to put as many pen dots into each of the targets as possible by moving back and forth between two targets. It is important to tap as fast as possible while maintaining accurate performance at all times. However, it is also important to try to stay inside of the targets at all times. Although misses will occur, in order to adequately test Fitts' Law there should be no more errors made in the most difficult task than there are in the easiest tasks. If the number of errors exceeds more than 5% of the pencil dots, the trial should be done again. A trial lasts 10 seconds.

If you have another person there - they can be your timer. If not, try to time yourself honestly and accurately.

Set up:

• Using the table below- draw boxes that are the appropriate distance/width ratio. 3 times per trial for 24 total trials. You may have to have 24 different set ups or you could try using different color markers to calculate your new number.

Data recording:

1. The data record should record your number of taps for each trial for each index of difficulty (ID) on the Individual Data Sheet.

IndividualData recording: (10pts)

Record the total number of taps for each ID target pair for each trial. Then calculate the Median. Remember, redo trials that have more than 5% error from the first Trial (i.e., if you miss the box 3/10 times in trial 1 - you must redo any trial that misses more than that in subsequent trials.)

ID	D/W	Trial 1	Trial 2	Trial 3	Median	(Errors for 1, 2, 3)
1	2/2
2a	2/1
2b	4/2
3a	4/1
3b	8/2
4a	8/1
4b	16/2
5	16/1

1. For each ID, calculate the average movement time (in msec) for a singlemovement of the reciprocal tapping task.

Example:

Data set

D/W	Trial 1	Trial 2	Trial 3	Mean
2/2	38	46	50	44.67 taps in 10 sec.

1. By dividing each number of taps by 10, which will give you the number of taps per sec during the 10 sec trial.

44.67/10= 4.67

1. Take the inverse of this number, which is 1/X (where X is the mean number of taps).

1/4.467 = .224

1. Then, multiply this number by 1000 to obtain the average movement time in msec.

.224 X 1000 = 224 movement time in msec

1. Record this mean movement time in the following table. (4pts)

ID	D/W	Median Taps	No. Taps/sec (Mdntaps/10)	Mean Movement time (msec)
1	2/2
2a	2/1
2b	4/2
3a	4/1
3b	8/2
4a	8/1
4b	16/2
5	16/1

1. Record your Mean Movement Time results for each ID on the Group Data Sheet.

Lab questions: (6pts)

1. Describe how the results of this lab relate to Fitts' Law.

2. Were there systematic differences in the group data between tasks that shared the same ID? If so, what might these differences indicate?

3. How do the results of this lab show a speed-accuracy tradeoff as it relates to task characteristics?

4. Describe a skill performance situation in which task difficulty is influenced by target size and distance relationships.