Topics: Slope, Standard deviation, Arithmetic mean Pages: 7 (1307 words) Published: February 18, 2014
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Lab #3- APA3120 Information Processing Speed-Accuracy Trade-off

Introduction

Paul Fitts’ was the first to discover the relationship between the speed of movement and accuracy requirements. This has become one of the most fundamental principles of motor control. Fitts claims a relationship between task difficulty and movement time. He quantified task difficulty as “index of difficulty” which consists of the ratio of twice the amplitude over width of the target (2Amplitude/Width). This relationship states that when movement amplitude decreases or when target width increases movement time is shorter (Fitts, 1954). Fitts found that the relationship between amplitude and width was given by the equation: MT= a + b[log2(2A/W)]. The empirical constants a and b represent the y-intercept and the slope (Schmidt & Lee, 2011).

The purpose of this laboratory was to investigate the linear relationship between the index of difficulty and movement time. Furthermore, the target width-amplitude relationship was observed. The goal was to determine the effect of task difficulty on movement time. Based on Fitts’ law, it was hypothesized that the relationship between movement time and index of difficulty would increase linearly.

The laboratory consisted of six trials, each consisted of a different movement amplitude to target width relationship. Refer to lab protocol for exact measurements. For each trial, the participant had to move a stylus between two targets, moving as fast as possible while maintaining accuracy. Each trial consisted of a fifteen second period, the number of total taps was recorded. Results from five participants were taken. Results represented movement times for index’s of difficulties of 1-4. The index of difficulty was an independent variable, movement time the dependent variable. It was expected that movement time was to increase with an increase in index of difficulty.

Results

Table 1

Mean movement time in milliseconds for six different conditions (n=5). Index of difficulty and standard deviation included. Condition
ID
Subject 1
Subject 2
Subject 3
Subject 4
Subject 5
Mean
SD
1
2
211,3
180,7
157,9
156,3
133,9
168,0
29,3
2
3
194,8
189,9
166,7
154,6
135,1
168,2
24,8
3
4
333,3
375
170,5
241,9
137,6
251,0
101,9
4
1
182,9
147,1
129,3
141,5
127,1
145,6
22,4
5
2
181,7
161,3
135,1
140,2
128,2
149,3
21,9
6
3
189,9
157,9
164,8
153,1
131,6
159,4
21,1
Note. Index of difficulty = log2(2A/W)

Table 2

Mean movement time in milliseconds of each subject for six different conditions (n=5). Standard deviation presented. Subject:
1
2
3
4
5
Mean MT:
216
212
154.1
164.6
132.3
SD:
58.6
86.2
17.51
38.5
4.07

Figure 1. Group mean of movement time (ms) as a function of six conditions (n=5). The six conditions each had different movement amplitude to target width relationship. Standard errors are represented in the figure by the error bars attached to each column.

Figure 2. Individual movement time (ms) as a function of the index of difficulty [log2(2A/W)]. Line of best fit had an equation of y= 3.2818 + 124.05, y= movement time and x= index of difficulty.

Figure 3 Group mean movement time (ms) as a function of the index of difficulty [log2(2A/W)]. Line of best fit had an equation of y= 29.873 + 99.025, y= movement time and x= index of difficulty.

Expected mean movement time for an index of difficulty of 6 was calculated using the equation from the line of best fit: y= 3.2818x + 124.05. The expected movement time was 143.7 milliseconds. A paired t-test was used to compare group mean movement times between the condition three and condition four results. The t-test revealed no significant difference, t(4)= 2.73,p= 0.0524, such that movement time’s of condition three (mean= 252 ms , SD= 101.9) was slower than movement time’s of condition...

References: Fitts, P.M. (1954). The information capacity of the human motor system in controlling the amplitude of movement. Journal of Experimental Psychology, 47, 381-391.
Michmizos, K., & Krebs, H. (2013, November 23). Pointing with the ankle: the speed-accuracy trade-off.. Pub Med. Retrieved February 1, 2014, from http://www.ncbi.nlm.nih.gov/pubmed/2427140
Schmidt, R., & Lee, T. (2011). Principles of Speed and Accuracy . Motor control and learning: a behavioral emphasis (pp. 223-261). Champaign, Ill.: Human Kinetics Publishers.