CHI 2009b: The Performance of Touch Screen Soft Buttons

Seungyon "Claire" Lee
Georgia Institute of Technology
Atlanta, Georgia

Shumin Zhai
IBM Almaden Research Center
San Jose, California

SUMMARY:
     This paper discusses the performance of soft buttons on different types of touchscreens.   A soft button refers to any button that is generated by the hardware.  A hard button is an actual hardware button either on the screen or on an input device.  The advantage of soft buttons is they can appear and disappear according to the user's context.  The disadvantage of soft buttons is that it is difficult to give user feedback as to whether or not it is being pressed.
     A previous study at the University of Maryland showed that when the cursor was placed 1/2 inch above the finger tip, less errors were made.  More recent studies have shown complex techniques that amplify the fingertips movement and placement which is a tradeoff for detection speed.
    One issue the authors plan to investigate is the accuracy of a soft button click with a stylus versus the accuracy of a fingertip.   Another issue is button size.  A previous study at grocery stores suggested 22mm as the minimum suggested size for a soft button.

The authors performed tests on five products:

• Apple Iphone (capacitive sensor, finger)
• HTC Touch phone (resistive sensor, finger or stylus)
• Canon DK10i (calculator with hard buttons)
• HP iPAQ 6315 smartphone (resistive sensor, finger or stylus, big like calculator)
• Nokia 6269 (mass market hard buttons) 

Thirteen users were studied. 7 of them used touchscreen devices on a daily bases.

Experiment 1: Operating Mode and Feedback

• Enter a multiplication operation, required 8 digits and 2 operator (x,=)
• Tested  Canon calculator vs. HP smartphone
• Tested 4 different categories of feedback
• No feedback
• Audio feedback
• Tactile feedback
• Audio + tactile feedback
• Speed was similar in both cases
• Error rate was high with no feedback for the touch screen
• Audio + tactile feedback isn't necessary, just choose one

Experiment 2: Contact Force vs. Force Activation

• Iphone, HTC Touch, Nokia 6260
• Task was to dial 7 digit phone numbers
• In this experiment very few errors were made, speed was similar
• There wasn't a significant difference in any of the phones at this task

 Experiment 3: Button Size and Mode of Activation

• Tested soft screen button sizes on the iPhone(contact) and HTC Touch(force)
• User had to enter a number, performance was measured by clicks per second
• Two phones had similar results
• Showed wide buttons(7.2mm) had better performance(clicks per second) than narrow buttons(4.9mm)

In general there was no difference between the two types of touchscreens.  I believe they should have used a phone that has a hard flip out keyboard.  They also should have tested sending SMS messages in one of their experiments.