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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


The following sections present thumb length and keypad layout, thumb circumference and key size, thumb circumference and space between keys, and thumb circumference and users' texting satisfaction.

Thumb length and keypad layout

A significant negative correlation was found between thumb length and users' satisfaction towards keypad layout. This could be due to the 4 x 3 layout used by the mobile phones in this study and in most of the mobile phones in the market. It was found that subjects with longer thumbs find it difficult to reach keys 3, 6, 9, and pound (#), which are all placed at the right-most column of the keypad (see Figure 1). Eleven subjects (thumb length ranged between 6.0 to 7.0 cm) commented that they have to practically bend their thumbs or readjust the placement of the phone on their palms in order to press these keys, especially for the pound key (#) which is at the bottom corner of the phone. The motion of adjusting their hands and thumbs to accommodate the keypad layout causes dissatisfaction among these users. When prodded further, four of them stated that continuously bending their thumbs has caused discomfort at the first joint of their thumb (from the tip), especially after continuously texting. An increase in texting may contribute to a rise in repetitive strain injury (RSI) in thumbs, especially among the young users. Bronwyn Clifford, of the Association of Chartered Physiotherapists in Occupational Health and Ergonomics (ACPOHE), mentioned that too much texting can result in pain and swelling of the thumbs and wrists as the thumb is good at grasping but not good for repetitive movement (Cannon, 2005). Balakrishnan et al. (2005) and Yeow and Balakrishnan (2007) have also reported that some of their subjects have experienced pain in their thumbs after prolonged texting; however, their study was based on a small sample size and disregarded thumb measurements.

Thumb circumference and key size

As mobile phones shrink in size, the key size shrinks as well. This further complicates the problem of texting among users with large thumbs. Thumb circumference was found to significantly correlate negatively to key size. This confirmed that as a user's thumb size increased their satisfaction decreased towards key size satisfaction. Large thumbed users (thumb circumference ranged between 6.5 to 7.5 cm) found it difficult to make multiple key presses on tiny keys. This was further aggravated by the limited space between keys. These users tended to accidentally hit the wrong keys when entering messages. Having to correct the errors caused frustrations among these users, hence decreasing their satisfaction with respect to key size. This finding was consistent with Soriano et al. (2005) who reported that four out of five male participants in their study claimed that the size of the keys became an issue when texting, especially among those participants with larger fingers. However, the researchers did not take any finger measurements in their study. Small key sizes were also reported as one of the mobile phone usability problems by Axup et al. (2005) and Ornella and Stephanie (2006); however, neither study took any thumb measurement details into consideration. Anderson (2005) reported that any tool that involves a struggle to be used earns a D or worse for usability. A common criticism is that mobile phones have become too small causing aim and accuracy problems when adult fingers use child-sized buttons.

Thumb circumference and space between keys

A similar correlation was found between thumb circumference and space between keys, with both significantly correlating negatively to each other. This showed that subjects' satisfaction decreased as their thumb circumference increased. Miniaturization of mobile phones causes the keys to be placed closely together. This results in a very limited space or no space at all between the keys. Large thumbed users found texting a tedious task due to the close placement of the keys, which was further aggravated by the tiny key size. Twenty-two subjects (thumb circumference ranged between 6.5 to 7.5 cm) commented that they tended to hit the neighbouring keys accidentally while texting, especially when done in a hurry or while in motion (e.g., walking or talking). It can be a frustrating task as they had to waste their time correcting the errors instead of texting efficiently. Moreover, they mentioned that they needed to constantly focus on the screen to make sure they had pressed the correct key, which eliminated the possibility of eyes-free input among the large thumbed users. Frequently having to correct their errors hindered these users from adopting SMS at times or to use it only to send simple, short messages. Ornella and Stephanie (2006) also found limited spaces between the keys to be a problem among the older mobile phone users (60 to 80 years old); however, no thumb measurements were taken into consideration. Due to this dissatisfaction with texting, subjects tend to make phone calls that are faster instead of making slow key presses to send text messages.

Thumb circumference and users' texting satisfaction

Finally, a significant negative correlation was found between thumb circumference and users' texting satisfaction, indicating an increase in thumb circumference will significantly decrease texting satisfaction. This confirmed the previous findings whereby users' satisfaction towards key size and space between keys decreased as the thumb circumference increased. It can be concluded that user's dissatisfaction towards key size and space between keys affect user's texting satisfaction. As stated previously, large thumb sized users found it difficult to make multiple key presses via the tiny keys that are placed close to one another, and thus results in an increased error and correction rates. This caused the users to be frustrated in using the keypad to text.

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