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A Study of the Effect of Thumb Sizes on Mobile Phone Texting Satisfaction

Vimala Balakrishnan and Paul, H.P. Yeow

Journal of Usability Studies, Volume 3, Issue 3, May 2008, pp. 118-128

Article Contents


Keypad Design

Standard ISO 12-key keypad design

Figure 1. Standard ISO 12-key keypad design

The standard keypad design on many mobile phones consists between 12 to 15 keys. Each key contains multiple letters (see Figure 1). These keys are used to enter text, symbols, numbers, and punctuations. Users repetitively press keys for text input because each key is mapped to between one to five characters. The most popular forms of text input on a standard 12-key mobile phone are multitap or predictive text entry.

In a multitap system the user presses the key multiple times to make a letter selection. For example, the key 2 is loaded with the letters A, B, and C, thus if a user wants to enter a C, then he or she has to press the key three times (222) successively as C is the third letter placed on the key. The process of texting becomes more complicated when the intended letters are placed on the same key. For example, to text the word cab the user must press the 2 key using the following pattern: 222 (pause) 2 (pause) 22. To select the correct letter on the key, the user must pause to determine the correct letter. Most of the mobile phones employ a time-out process in which the user waits for a specified time (typically one to two seconds) before attempting to enter the next letter, which is why multitap is often criticized for being slow (Mackenzie, 2002).

On the other hand, predictive text entry uses a dictionary to disambiguate user's input. Just like multitap, multiple keys correspond to the same key. However, keys are pressed only once. Possible words are continuously guessed as letters are entered and users can cycle through these words via a next key. Though this method is faster than multitap (James and Reischel, 2001), it can be quite frustrating and slow when the phone doesn't recognize the words that are entered (Starner, 2004). Predictive text entry is a particular problem when using an English keypad to enter non-English text. Moreover, it is also impossible to enter numerals, acronyms, or any combinations of letters and numerals (e.g., l8r for later). Users also have to visually monitor the display to resolve ambiguities, unlike multitap that can be operated "eyes free" by experts (Mackenzie et al., 2001).

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