Data entry task in mobile computing: effects of vibration, display colour, and user age
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The sensor of the vibration level meter was kept at a specially designed platform which did not affect the impact of vibration and the display was kept in front of the
experimenter to allow him to constantly monitor the level of vibration. The mean value of the angle between eye level and the centre of the screen of
the lap-top for all the subjects was kept at approximately 15° SD = 3.16°. The temperature of the vehicle was maintained at approximately 26 ± 3 °C. While
performing the experimental task the level of vibration in the driving environment was kept at a specified value by constantly monitoring the level of vibration in the x,
y and z directions and running the vehicle at the appropriate speed.
Before the actual experiment in the test vehicle, a trial session was arranged; this was done for two purposes. The first purpose was to familiarize the subjects
with the experimental procedure; the second was to determine the reaction of the subject in connection with the recorded text input in the audio-cassette that was
replayed during the experimental session. Subjects did not adapt easily to this style of stimulus presentation style; therefore one experimenter sat beside of the subject
and read the text to be entered. Out of 42 subjects 32 preferred this style of stimulus presentation but requested slow speed reading so that the text entry task could be
completed without difficulty. Subjects were required to sit on the vehicle seat without back rest with the two hands on the keyboard as was observed to be the
practice of the end-users while working on VDUs .Before performing the experimental task data entry, the subject sat in the vehicle at a prespecified level of
vibration for 60 minute and then the stimulus was presented to him. Each text entry task required 10 minutes. After completion of the task at each level of vibration
separately, the content entered was saved with the name of the subject and later downloaded later to check it for errors. Subjects were allowed to rest for 30 min
between successive trials; during this period they were isolated from the driving environment. Two textbackground color employed in this experiment: black
characters on white background and white characters on black background. The experimental task was repeated for both textbackground colour combinations.
2.3. Experimental Set-up
Experimental investigations were carried out in a real life driving environment on a passenger car Waja manufactured in Malaysia. This particular type of car was
chosen because ~ 65 of Malaysian people use this car. This car has a 1597-cc S4PH 4- cylinder 16-valve DOHC engine that has a maximum power of 82 kW at 6000 rpm,
a maximum torque of 148 Nm at 4000 rpm, and a multi-point fuel injection system. The bore of this engine is 76.0 mm and its stroke is 88.0 mm. The front suspension
uses a MacPherson strut with a stabilizer bar; the rear suspension uses Multilink with stabilizer bar. The car is 4465 mm long, 1740 mm wide, 1420 mm high and has
a wheelbase of 2600 mm. Its maximum speed is 190 kmh.
The experimental setup Figure 1 was comprised several sub-systems: a vibration level meter Bruel Kjaer Deltatron, Type 4504, a 12-volt battery for
energizing the amplifier Bruel Kjaer, Type 2260, a digital lap-top Armoda 1500 Compaq.
ZULQUERNAIN MALLICK AJE Vol.9 2009 19-32
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2.4. Error and error handling
Another important dimension to consider in such experiments is the error rate. What matters is the effective speed, or speed after correction. Different methods
of handling errors can be used; each has different implications to the text input study result. Some leave errors in the text and report them separately MacKenzie
and Zhang, 1999. Others do not allow errors, e.g. the testing program will not proceed until the correct character is entered Zhai et al., 2002. Yet others require
the participants to correct their errors, and measure their effective speed including error correction. The amount of time needed to correct errors depends on the design
of the error correction mechanism; hence no set rules exist to compare tradeoffs between error and speed. Typing accuracy is defined as the percentage of characters
correctly typed. In this experiment, error was counted on the basis of characters typed correctly. For example:
z Given Text: Please come to meet me at the play ground where match will
be played. z
Entered Text: Please come to meet me at the flat ground where match well be played.
The given sentence has 55 characters without spaces and the entered text has two errors, one in the word ‘Please’ and the second in the word ‘Play’. Therefore the
error rate is 255 x 100 = 3.63 and typing accuracy is 100 - 3.63 = 96.37. Data entry task performance is often measured in words per minute WPM. In
the present research performance of operators were measured in NCEPMWS. This style was adopted to nullify the subjects’ habit of leaving single or double spacing
between words while performing data entry task.
POWER SUPPLY
LAPTOP SCREEN KEY BOARD
VIBRATION LEVEL METER
EXPERIMENTER SUBJECT
SEAT
340 mm 340 mm
5 3
9 7
5 m
m
Figure 1. Schematic diagram of experimental set-up employed in all the experimental
investigations undertaken in the present work without vibration zero level and vibration with vibration levels of 0.85 and 1.65 ms
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in the HCI environment.
