Hand Anthropometry and SMS Satisfaction

Vimala Balakrishnan and Paul H. P. Yeow

Vimala Balakrishnan and Paul H. P. Yeow, 2008. Hand Anthropometry and SMS Satisfaction. Journal of Applied Sciences, 8: 816-822.

DOI: 10.3923/jas.2008.816.822

URL: https://scialert.net/abstract/?doi=jas.2008.816.822

MATERIALS AND METHODS

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Text entry on mobile phones: The standard keypad layout on many mobile phones consists between 12-15 keys that are overloaded (Fig. 1). These keys are used to enter text, symbols, numbers and punctuation. Each key is mapped to between three to four characters, resulting in the users having to make repetitive key presses for text input. The most popular forms of text input on a standard 12-key mobile phone are either multitap or predictive text entry.

Multitap: Successive key presses are made on different or the same keys to determine the intended letters. For example, to enter SMS, the user would press 7777 (to get the fourth letter on key-7), then 6 and finally 7777. The break between two letters entered with the same key is normally indicated by a pause (1-2 sec). Multitap is simple and unambiguous; however it can be slow (Mackenzie, 2002).

Predictive text entry: With predictive text entry, the mobile phone displays the most likely word for the sequence of keys pressed since the last space character. If the predicted word is incorrect, the user will have to scroll through the alternative words. However, if the word is not recognized at all, then the user must delete their input and enter the word via multitap. This can be quite frustrating. 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).

Design of study: Figure 2 shows the theoretical framework used in this study. SMS satisfaction acts as the dependent variable whereas mobile phone and keypad design factors are the independent variables, tested against hand anthropometry.

Both the independent variables were chosen as they represent the physical aspects of a mobile phone that directly relate to hand anthropometry. A mobile phone is held in one or two hands and messages are entered via the keypads using one or two thumbs. Moreover, a lot of work has identified these two factors as some of the usability issues of mobile phones (Axup et al., 2005; Soriano et al., 2005; Balakrishnan et al., 2005). Table 1 and 2 show the different factors tested for the independent variables.

All the design factors for mobile phone and keypad were identified from studies conducted to determine some of the critical mobile phone design features to users` satisfaction (Ling et al., 2007; Han et al., 2004; Yun et al., 2003).

Fig. 1:	Standard ISO 12-key keypad design

Fig. 2:	Theoretical framework

Table 1:	Mobile phone design factors

Table 2:	Keypad design factors

Other studies that have reported on issues related to some of these features are: problems related to tiny keys (Soriano et al., 2005; Kurniawan et al., 2006),problems related to key size and space between keys (Balakrishnan et al., 2005; Ornella and Stephanie, 2006) and issues on the shrinking size of mobile phones (Anderson, 2005; Croasmun, 2002). None of these studies, however, took hand anthropometry into consideration.

Subjects: One hundred and ten youth ranging between 17-25 years old were interviewed (mean = 21.5 years, SD = 1.64). The majority of them (84/110) were recruited from a local university and the rest were selected from public places (mall, public library etc.). All the subjects had some experience in using SMS, with an average of 3.8 years and SD = 1.19. All the subjects also used their thumbs to compose messages. 80.9% (89/110) of the subjects used multitap for text entry, 11.8% (13/110) used both multitap and predictive text entry interchangeably and only 7.3% (8/110) used predictive text entry. The mobile phones used in this study were by some of the popular brands, namely Nokia with 66.4% of the subjects owning this phone. This was followed by Samsung (12.7%), Motorola (9.1%), Sony Erickson (8.2%), Siemens (1.8%) and Alcatel (1.8%).

Hand anthropometry: Different hand anthropometries were measured for mobile phone and keypad design factors. Users with small hands might find it difficult to hold large mobile phones and users with large hands might find it difficult to hold small mobile phones, thus hand breadth was measured to test if it affects users` satisfaction towards mobile phone design factors. Hand breadth was measured at the distal ends of the metacarpal bones (the joints of index finger to the little finger) with the hand held straight and flat (Fig. 3a). Thumb length might affect users` reachability of the keys whereas users with large thumbs might find it cumbersome keying in messages via the tiny keys. Thumb length was measured from the second joint of the thumb to the tip of the thumb whereas thumb circumference was measured at the widest point of the thumb (Fig. 3b). These measurements were taken based on the physical definitions used by Vasu and Mital (2000). Measurement tapes were used and all the readings were taken twice to obtain the average measurements.

Questionnaire: An interview questionnaire was developed in English with two major sections: Section A to obtain the demographic profile of the subjects (gender, hand measurements, hand used to text etc.) whereas Section B is for the subjects to rate their satisfaction/dissatisfaction levels to statements using Likert`s five-point scale, whereby 1 means Strongly dissatisfied, 2 means Dissatisfied, 3 means Neutral, 4 means Satisfied and 5 means Strongly Satisfied.

Interviews: Face-to-face interviews were conducted using the above questionnaire on a one-to-one basis, beginning with the subjects filling in their background information, which includes their age, gender, finger(s) used in composing SMS and so forth. The interviewer then measured the hand and thumb sizes, based on the dominant hands. Subjects were encouraged to give comments, opinions and suggestions.

Fig. 3a:	Hand breadth measurement

Fig. 3b:	Thumb measurements

All verbal comments were noted by the interviewer. Each interview session lasted for about 30 min. Two interviewers participated in these exercise that took almost eight weeks to complete.

Statistical tests: Statistical Package for the Social Sciences (SPSS) software was used to test the statistical significant difference(s) of the variables. Hand-size groups were tested against mobile phone design factors whereas thumb length and circumference were tested against keypad design factors. Analysis of variance (ANOVA), analysis of covariance (ANCOVA), Tukey Post-Hoc analysis and Pearson correlations were used to analyze the collected data.

RESULTS

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The five-point Likert scale structured interview questionnaire has an acceptable level of internal consistency with a Cronbach`s alpha value of 0.73. Table 3 shows the summary of hand anthropometry statistics based on gender. Three hand-size groups (small, medium and large) were defined based on the hand breadth: for males, <8.8 cm is small, 8.8-9.2 cm is medium and >9.2 cm is large; for females, <7.3 cm is small, 7.3-7.7 cm is medium and >7.7 cm is large (You et al.,2005). The overall number of subjects for each hand-size groups is: 28 small, 43 medium and 39 large.

Table 3:	Hand anthropometry statistics
Note: Dominant hand measurements only

Table 4:	Mobile phone design factors satisfaction, based on hand-size
*: Significant at p<0.05

Table 5:	Keypad design factors satisfaction, based on thumb circumference and length
*: Significant at p<0.05

Table 4 shows that hand-size significantly affects users` satisfaction with respect to mobile phone dimensions. Tukey post-hoc analysis revealed that small hand-sized subjects are significantly more satisfied than large hand-sized subjects (p = 0.001).

Table 5 shows that thumb circumference significantly affects users` satisfaction towards key size and space between keys. A significant correlation was found between thumb circumference and key size (p = 0.001, r = -0.309) and with space between keys (p = 0.004, r = -0.272). Thumb length was found to be significant only for keypad layout, both also significantly correlated to each other (p = 0.042, r = -0.194).

DISCUSSION

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According to the p-values in Table 4, hand-size significantly affects users` satisfaction towards mobile phone dimension. Smaller hand-sized subjects are more satisfied with the mobile phone dimension than large hand-sized subjects. A good and comfortable grip is necessary to enable one to compose messages in an efficient manner, however, this is not possible for users with large hands who have to struggle messaging while holding the small mobile phones. The users find it awkward pressing the keys while holding the small and slim mobile phones in their large hands. This statement is supported by Margaret Head, the President of Ergonomics Society of Australia who said that mobile phones are more suited to a young child than a grown adult (Croasmun, 2004). Nine large hand-sized subjects mentioned that they only use SMS to send very short messages; otherwise they prefer making calls as they can carry conversations by holding their phones (no fumbling with fingers and keys required) or by using hands-free gadgets. Mobile phone dimensions were also found as one the features impacting overall satisfactions of mobile phone users by Ling et al. (2007); however, no hand-size measurements were included.

In Table 5, thumb length significantly affects keypad layout satisfaction, negatively correlating with each other. This could be due to the 4x3 layout used by the mobile phones in this study and in most of the mobile phones in the market as well. It was found that subjects with longer thumbs find it difficult to reach key -3, 6, 9 and #, which are all placed at the right-most column of the keypad (Fig. 1). Eleven subjects 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 key # which is at the bottom corner of the phone. The motion of adjusting the hand and thumb to accommodate the keypad layout causes dissatisfaction among these users. When prodded further, four of them stated that continuously bending their thumb have caused discomfort at the first joint of their thumb, especially after continuously messaging. Increase in messaging 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 messaging can result in pain and swelling of the tendons at the base of the thumb and wrist as the thumb is good at grasping but not good for repetitive movement (Cannon, 2005). The majority of the subjects (64/110) stated that the keys placed in the middle of the keypad (sweet spot) were the easiest to be accessed and most comfortable as well.

Thumb circumference was found to significantly affect users` satisfaction towards key size (Table 5). As mobile phones shrink in size, the key size shrinks as well. This further complicates the problem of messaging among users with large thumbs. Thumb circumference was also found to significantly correlate negatively to key size. This confirms that as users` thumb increases in size, their satisfaction decreases towards key size satisfaction. Large thumbed users find it difficult to make multiple key presses on the tiny keys. This is further aggravated by the limited or no space between keys. These users tend to accidentally hit the wrong keys when entering messages. Having to correct the errors cause frustrations among these users, hence decreasing their satisfaction with respect to key size. This finding is 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 messaging especially among those 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 none took anthropometric 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 to suffer when adult hands finger child-sized buttons.

Thumb circumference also significantly affects users` satisfaction with respect to space between keys (Table 5) and both correlating negatively to each other. Large thumbed users are more dissatisfied with the limited space between the keys. Miniaturization of the mobile phones also causes the keys to be placed closely together; hence limiting the space between the keys. Large thumbed users find messaging a tedious task due to the close placement of the keys, which is further aggravated by the tiny key size. Thirty two subjects commented that they tend to hit the neighbouring keys accidentally while messaging, especially when it is done in a hurry or while in motion (e.g., walking, talking etc.). It can be a frustrating task as they have to waste their time correcting the errors instead of messaging efficiently. Moreover, they also mentioned that they need to constantly focus on the screen to make sure they have pressed the correct key; hence eliminating the possibility of ‘eyes-free` input among the large thumbed users. Frequently having to correct their errors hinders these users from adopting SMS at times or to use it only when it is deemed necessary, for example to send simple and short messages, especially single line messages. This is especially true among the male users, who generally have larger thumbs than females (Table 3). Ornella and Stephanie (2006) also found limited spaces between the keys to be a problem among the elderly mobile phone users (60-80 years old); however no anthropometric measurements were taken into consideration. Soriano et al. (2005) found that spacing between the keys became an issue especially for participants with large fingers, based on their survey among middle aged-users. Moreover, subjects with larger thumbs tend to be more careful when making key presses to avoid making unwanted errors and this increases the time spent on composing a message. Due to this, subjects tend to make phone calls that are faster instead of making slow key presses to message.

Key size and space between keys seem to be the major obstacles among users with large thumbs. The problem is further complicated for large hand-sized users who also have to struggle holding the small mobile phones while sending SMS. This shows that mobile phone users are not satisfied with the mobile phone and keypad designs. Interestingly, the dissatisfaction towards key size and space between keys exists regardless of the text entry method used as both require users to make key presses via the keypads. Multitap technique is often criticized for being slow. An experiment using a mobile phone found that experts and novices reached about 8 words per minute (wpm) with multitap (James and Reischel, 2001). In 2003, the world`s fastest mobile texter typed 29 wpm using multitap technique, which is more than six times slower than the Guinness record of 192 wpm for the desktop QWERTY keyboard (Starner, 2004). Multitap requires users to make successive key presses to enter characters and numbers. This complicates the task of messaging as users with large thumbs find it cumbersome to make key presses on the closely arranged tiny keys. On the other hand, the predictive text entry requires users to cycle through possible words as they are predicted by the mobile phone. Apart from having to make key presses, users also have to visually monitor the screen to select the desired word. Moreover, users still need to switch to multitap to enter numerals. These text entry problems coupled with poor design of keypads greatly cause dissatisfaction among mobile phone users to SMS, especially among users with larger thumbs.

CONCLUSION

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The effect of hand anthropometry on SMS satisfaction was studied based on hand-size, thumb length and circumference measurements. These measurements were tested against mobile phone and keypad design factors. The following results were drawn:

•	Smaller hand-sized users are more satisfied with the mobile phone dimensions than larger hand-sized users, with regards to holding the mobile phone and messaging at the same time,
•	The increase of thumb circumference decreases users` satisfactions towards key size and space between keys as messaging becomes tedious due to accidentally hitting the wrong keys,
•	The increase of thumb length decreases users` satisfactions towards keypad layout as accessing some of the keys becomes difficult and sometimes causes pain in the thumbs,
•	All the above problems were reported regardless of the text entry method used to enter text.

It can be concluded that hand anthropometry definitely affects mobile phone users` SMS satisfaction. Customized mobile phones have been designed to suit the elder people (Croasmun, 2005) and also kids (Budnick, 2005) to improve usability and increase satisfactions. With this in mind, the results from this study can be used to design customized mobile phones that suit users with larger hands and thumbs or even enhance the current designs to increase the satisfaction among all mobile phone users.