Stimuli and general experimental procedure

Cognitive performance of grass trimmers in noisy conditions - the effect of user age 37 Three tasks were assigned, which had different levels of difficulty. In all tasks, subjects were required to switch on the same color of LED at Circuit-2 by observing the glowing LED and its color at Circuit-1. In the simplest task Difficulty Index 1, subjects were required to observe the color of the LED on Circuit-1, and to light the LED on Circuit-2 that had the same color; this trial provided a simple reaction time. In the second task Difficulty Index 2 two LEDs of different color were lit on Circuit-1, and subjects were required to switch on an LED of one of the two colors on Circuit-2. In the third task Difficulty Index 3 three LEDs of different color were lit and subjects were required to switch on an LED of one these colors on Circuit-2. The locations of the LEDs on Circuit-2 were clearly labeled. Subjects were trained during a trial session to switch on the appropriate LED. All reaction times in milliseconds and user errors were measured using an electronic device designed and developed for this purpose Appendix A However subjects committed a negligible number of errors 2. After the subject had taken his seat, the following steps were followed in order for both the training and experimental sessions. Step 1: A Start signal was given to the subject. Step 2: The stimulus was presented to the subjects by the experimenter. Step 3: The subject responded by switching on the appropriate LED on Circuit-2. Step 4: The reaction time was recorded by the experimenter. The experiment was conducted for three categories of subjects: Novice, Intermediate and Experienced workers. The noise during grass trimming operation in a real environment was recorded on an audio-cassette. The pre-recorded noise was subsequently played in the experimental chamber during the experiments. The noise level was maintained at a pre-specified value. Training was rigorous for older subjects so that they were made fully aware of the experimental procedure.

2.8. Experimental Design

In all the three studies undertaken in the present work, human performance dependent variable was measured in milliseconds for reaction time, choice reaction time and choice reaction time for difficulty index three. Independent variables were difficulty index, noise levels and subject age category. A three-factor repeated-measures analysis of variance ANOVA was used to analyze the data.

3. Results

Three experiments were conducted to study the effect of age on cognitive performance of subjects. ZULQUERNAIN MALLICK AJE Vol.9 2009 33-48 38

3.1. Experiment 1

In this experiment, the effect of age on cognitive performance was investigated under varying levels of difficulty index and noise levels for Novices. ANOVA Table 1 implied that subject age and difficulty index had statistically significant effects on subject performance. The relationship between the reaction time and noise was linear Figures 3, 4, with regression equations as follows. Notation RT DIi reaction time at difficulty index i B noise level Age-group 1 Novices RT DI1 = 500 +0.4 B 1 RT DI2 = 600 + 0.31 B 2 RT DI3 = 675 + 0.45 B 3 Age group2 Novices RT DI1 = 575 +0.36 B 4 RT DI2 = 650 - 0.13 B 5 RT DI3 = 694 + 0.48 B 6 Table 1. ANOVA Results when subjects performed the cognitive task Novices Source of variation df MS F P Between Subjects 13 9153.71 Age group A 1 1221495.41 69.37 0.01 Subjects within groups Error 1 12 17607.80 Within Subjects 154 23083.95 Noise levels B 3 51205.10 0.933 0.32 A x B 3 11686.46 0.21 0.78 B x Subjects within groups Error 2 36 54825.21 0.01 Difficulty index C 2 3093389.23 101.41 0.00 A x C 2 3650.89 0.11 0.87 C x Subjects within groups Error 3 24 30501.67 B x C 6 4558.09 0.374 0.88 A x B x C 6 1390.42 0.11 0.91 B x C x Subjects within groups 72 12187.67