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					                                                            Speed Accuracy Trade-Off 1

Running Head: SPEED AND ACCURACY TRADE-OFF IN MANUAL AIMING TASKS
AND PURSUIT ACCURACY




      Speed and Accuracy Trade-Off in Manual Aiming Tasks and Pursuit Accuracy

                                        by
                                 Simone Kortbeek
                                    200801836

                                 A laboratory report
                             presented to Tara Artibello
                                    in HKIN 215
                              Intro to Motor Learning

                           Department of Human Kinetics
                            St. Francis Xavier University

                                 December 4, 2009
                                                                   Speed Accuracy Trade-Off 2

       In 1954 Paul M. Fitts found target acquisition time to have a linear relationship with the

information load or difficulty of a movement. This was discovered through an experiment now

dubbed Fitts’ pointing paradigm (Guiard & Beaudouin-Lafon, 2004). This was a variance of

Fitts’ initial prediction that movement time would be directly proportional to the information

load of a movement (Guiard & Beaudouin-Lafon, 2004). Since then, this widely applicable

finding has been implemented in a multitude of situations. Studies such as that of Seya and Mori

(2007) in which the trade-off between motor time and pursuit accuracy was studied have greatly

expanded upon Fitts’ law. This experiment was designed to test Fitts’ findings in a traditional

manner by way of a manual aiming task with variable complexity or difficulty of movement.

There should be an increase in motor time as the difficulty of the movement increases. Thus an

increase in motor time will correspond with a decrease in target size or an increase in the

distance between targets.

                                             Method

Subjects

       The participant in this study was an undergraduate student at St. Francis Xavier

University who was enrolled in HKIN 215. This single participant took part in all four conditions

of the study.

Apparatus

       Materials used in this experiment were target sheets, one for each condition, a pen and a

stopwatch.

Procedure

       The participant performed three trials for each condition, in random order. Each condition

had a varying degree of difficulty depending upon the distance between the targets and the width
                                                                    Speed Accuracy Trade-Off 3

of the targets. Each trial had a duration of ten seconds, which was timed by the experimenter.

The experimenter signaled the beginning and the end of each trial with the words “ready - go”

and “stop” respectively. During each trial the subject attempted to tap the pen within the two

targets on the target sheet alternately as quickly as possible. Between each trial there was a rest

period equivalent to the time it took to count the taps within each of the two targets for that

particular trial. If the number of errors reached or exceeded five percent, the trial was repeated.

An error was considered to be a pen mark outside the designated target area on the target sheet.

Experimental Design

       In condition A the distance between targets on the target sheet was two centimeters and

the width of each of the two targets was two centimeters. In condition B the distance between

targets was four centimeters and the width of each target was two centimeters. In condition C the

distance between targets was eight centimeters and the width of each target was one centimeter.

In condition C the distance between targets was sixteen centimeters and the width of each target

was two centimeters. The independent variables in the study were both the distance between

targets and the width of each target. The dependant variable in the study was the cumulative

number of pen marks within the two targets.

                                               Results

       For condition A the number of taps per second was 4.1, the mean motor time for this

condition was 243.9. The number of taps per second for condition B was 3.5 and the mean motor

time was 285.7. The number of taps per second for condition C was 2.2 and the mean motor time

for this condition was 454.5. For condition D the number of taps per second was 2.5 and the

mean motor time was 400. For further data summary see Table 1. Mean motor time was
                                                                      Speed Accuracy Trade-Off 4

calculated by taking the inverse of the average number of taps per second then, multiplying this

number by 1000 to get the average motor time in milliseconds.

                                              Discussion

        Using target sheets to measure the number of taps per second and further more calculate

mean motor time, this study was able to evaluate the speed and accuracy trade-off of a manual

aiming task as described by Paul Fitts. The number of taps per second decreased as the distance

between targets increased and as the width of the targets decreased. Mean motor time increased

as distance between targets increased and as the width of the targets decreased. These results

confirm a trade-off between speed and accuracy in a manual aiming task. The data showed a

decrease in motor time as the difficulty or complexity of the task increased, either with an

increase in the distance between targets or a decrease in the width of the targets.

        Similar results were found in a study done by Seya and Mori (2007) in which the reaction

time to a visual target was measured during smooth pursuit of a moving fixation stimulus. Seya

and Mori (2007) found that participants displayed a decrease in pursuit gains as the velocity of

the fixation stimulus increased. A pursuit gain in this study was measured as the ratio of eye

velocity to the fixation stimulus velocity. From the decrease in pursuit gains it can be deduced

that participants may have reduced pursuit accuracy in order to achieve faster reaction times

(Seya & Mori, 2007). In other words there is a trade-off between pursuit accuracy and reaction

times (Seya & Mori, 2007). Murata and Iwase (2001) extended Fitts’ law further and attempted

to apply it to a three-dimensional pointing task in which the difficulty of the movement was

influenced by target size, distance to the target and direction to the target. In this study the task of

the subject was to point to a target as directed by the experimenter, with their right index finger.

There were significantly more variable movement times in this study as there was a significant
                                                                    Speed Accuracy Trade-Off 5

effect of movement direction on movement time (Murata & Iwase, 2001). Although the

conventional Fitt’s law was modified to increase the predictive power on motor time by the

addition of variables, specifically a directional term, the performance model is still applicable to

a certain degree to three-dimensional pointing tasks (Murata & Iwase, 2001).

       Tresilian, Plooy and Marinovic (2008) also confirmed Fitts’ prediction of a speed-

accuracy trade-off in a more complex movement situation by testing the belief that movement

time is typically greater when there is a greater accuracy requirement in an interceptive aiming

task. In this experiment subjects were required to move a hand held manipulandum along a

vertical track, into the path of a moving target. Thus in this case movement difficulty was

dependent upon a temporal accuracy requirement and a spatial accuracy requirement. The typical

response to a greater requirement of spatial accuracy was found to be longer duration

movements. This response was also found when subjects were required to hit smaller targets

(Tresilian et al., 2008). These findings support those of this study and support the notion that

longer motor times are a typical response to greater accuracy requirements.
                                                                 Speed Accuracy Trade-Off 6




                                           References

Guiard, Y., & Beaudouin-Lafon, M. (2004). Fitt’s law 50 years later: applications and

       contributions from human-computer interaction. International Journal of Human-

       Computer Studies, 61, 747-750.

Murata, A., & Iwase, H. (2001). Extending Fitt’s law to a three-dimensional pointing task.

       Human Movement Science, 20, 791-805.

Seya, Y., & Mori, S. (2007). Tradeoff Between Response Speed and Pursuit Accuracy. Motor

       Control, 11, 109-118.

Tresilian, J., Plooy, A., & Marinovic, W. (2009). Manual interception of moving targets in two

       dimensions: Performance and space-time accuracy. Brain Research, 1250, 202-217.
                                               Speed Accuracy Trade-Off 7

Table 1

Data Summary

   Condition   D(cm)/W(cm)   Median Taps   Number of Taps   Mean Motor
                                             Per Second     Time (msec)
          A        2/2           41              4.1           243.9
          B        4/2           35              3.5           285.7
          C        8/1           22              2.2           454.5
          D       16/2           25              2.5           400.0

				
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