touch key design

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Touch key design for one-handed thumb interaction with a mobile phone:

Effects of touch key size and touch key locationq

Yong S. Park, Sung H. Han*

Department of Industrial and Management Engineering, Pohang University of Science and Technology (POSTECH), San 31, Hyoja, Pohang 790-784, South Korea

a r t i c l e i n f o

Article history:

Received 7 October 2008

Received in revised form

4 August 2009

Accepted 4 August 2009

Available online 1 September 2009

Keywords:

Touch key size and location

One-handed thumb interaction

Usability

Mobile phones

a b s t r a c t

This study investigated effects of touch key sizes and locations on one-handed thumb input on a mobile

phone. Three different touch key sizes (i.e. square shape with 4 mm, 7 mm, and 10 mm wide) and

twenty-five locations were examined in an experiment. A total of thirty subjects participated in the

experiment in which they preformed a task of pressing a single target on a small touch screen. Two time-

related measures (first transition time and task completion time), number of errors, and subjective

satisfaction (pressing convenience) were collected in the experiment. The results revealed that the touch

key size of 7 mm and 10 mm provided the best performance for time-related measures, while the touch

key size of 10 mm only provided the best results for the other measures. In addition, the usability of

touch key locations was statistically analyzed. Touch key locations providing good usability (good

regions) were also identified for each measure. Recommendations were proposed for designing a touch

user interface on a mobile phone based on the results of this study.

Relevance to industry: The touch user interface is in the limelight of the handset industry. This study

conducted basic research to investigate the effects of touch key sizes and touch key locations for one-

handed interaction. The results of this study could be used for designing a touch user interface to

enhance the usability of mobile phones and other small devices with a touch screen as well.

2009 Elsevier B.V. All rights reserved.

1. Introduction

Touch screens are widely used for a variety of mobile devices

such as personal digital assistants (PDAs), portable multimedia

players (PMPs), and mobile phones since they are highly intuitive

and require little space to implement (Scott and Conzola, 1997).

In addition, touch interfaces are easy to adjust the design param-

eters, such as key size, spacing between keys, and location on the

screen (Colle and Hiszem, 2004). Mobile phones with a touch

screen replacing traditional keypads, e.g. the Apple iPhone, are

coming into the spotlight.

Mobile phones with a touch screen have to present touch keys

for user input as well as information for userphone interaction

(e.g. notification). However, they have small touch screens that

limit space for touch keys. Worse yet, they are unlikely to increase

the size of touch screens due to mobility and portability. Therefore,

it is important to design touch keys with the optimal/usable size.

Pfauth and Priest (1981) also reported touch key size as one of the

most important design factors.

Since 1980s, many studies have been conducted to investigate

usable touch key sizes. Only a few studies, however, have been

conducted for one-handed thumb interaction with a small touch

screen (Parhi et al., 2006), while most studies have been carried out

for interaction with a stylus as well as with an index finger

(Beringer, 1990; Colle and Hiszem, 2004; Hall et al., 1988; Martin,

1988; Scott and Conzola, 1997; Sears, 1991; Sears et al., 1993). Parhi

et al. (2006) examined touch key design implemented on a PDA.

They manipulated five different touch key sizes, i.e. 3.8 mm,

5.8 mm, 7.7 mm, 9.6 mm, and 11.5 mm square. However, thumb

movements on a touch screenwould be interfered by the PDA if the

hand size is not large enough, which is different from real phone

use. Note that most people move their thumbs freely when using

a mobile phone.

Two different approaches have been used to recommend

desirable touch key sizes (Colle and Hiszem, 2004). One approach is

to collect touch input fora spatial target and measure the minimum

size that captures a given percentage of touch input (Beringer,1990;

Hall et al., 1988; Sears, 1991). Hall et al. (1988) reported the

procedure of this approach in detail. This approach easily finds

minimum touch key sizes with which the users can press touch

q This work is based on an earlier work: Touch key design for target selection on

a mobile phone, Proceedings of the 10th mobile HCI conference, ACM, 2008.

http://doi.acm.org/10.1145/1409240.1409304.

* Corresponding author. Tel.: 82 54 279 2203; fax: 82 54 279 2870.

E-mail address: [email protected] (S.H. Han).

Contents lists available at ScienceDirect

International Journal of Industrial Ergonomics

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / e r g o n

0169-8141/$ see front matter 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.ergon.2009.08.002

International Journal of Industrial Ergonomics 40 (2010) 6876


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keys with accuracy of 95%, 99%, etc. With respect to accuracy only,

however, it could provide the minimum touch key sizes. The

approach is not applicable to analyzing other usability measures

such as task completion time and user preference. The other

approach is to manipulate touch key sizes experimentally and

compare them in terms of performance and subjective satisfaction

(Colle and Hiszem, 2004; Martin, 1988; Scott and Conzola, 1997;Sears et al., 1993). This approach can systematically analyze

usability levels of specific touch key sizes with respect to plural

usability measures, while it fails to identify minimum touch key

sizes with specific accuracy. This study adopted the second

approach and analyzed effects of touch key sizes on a variety of

usability measures including task completion time, accuracy, and

user preference.

The users tend to use only one hand when they use a mobile

device (Karlson et al., 2006). In other words, they hold a mobile

phone with one hand and interact with it using a thumb. In addi-

tion, they would use both hands only when the user interface

makes one hand interaction impossible or difficult. Touch key

locations as well as touch key sizes have been considered as an

important factor that could affect usability of one-handed thumb

interaction. Parhi et al. (2006) divided a PDA screen into 33 areas,

a total of nine areas, and found that central areas were more

preferred than the others in terms of subjective satisfaction.

However, the results for the nine areas are not enough to be applied

to a mobile phone interface, since mobile phones often provide

more than nine input elements simultaneously. For example, the

Apple iPhone can provide twenty items at a time in the home

screen. Park et al. (2008) examined a total of 25 touch key locations

with respect to accuracy and user satisfaction, which is an earlier

work of this study. However, it also did not include time-related

measures, important performance measures. To understand the

effects of touch key locations more clearly, it is necessary to

examine touch key locations from the overall usability perspectives

including task speed, task accuracy and user satisfaction.

This study aims to systematically investigate effects of touch key

sizes and touch key locations on the usability of a mobile phone.The specific objectives to achieve the goal include: (1) Comparing

different touch key sizes and identifying usable size to a mobile

phone, (2) finding out how pressing performance and user prefer-

ence changes according to touch key locations, (3) identifying

appropriate touch keylocations that provide good usability forone-

handed input on a mobile phone. In order to fulfill these objectives,

a human factors experiment was conducted, in which three touch

key sizes and twenty-five touch key locations were manipulated.

2. Methods

2.1. Subjects

A total of thirty right-handed Korean subjects participated in

a human factors experiment. Their age ranged from 18 to 28 years

old (average of 23.1 and standard deviation of 2.5). They had

normal vision and no problem to freely move their right thumbs.

Twenty of them had not used a mobile device with a touch screen

(e.g. personal digital assistants), while the others had experienced

for 1.2 years on the average.

The subjects hand and finger sizes were measured in terms of

three dimensions such as digit 1 interphalangeal joint breadth

(thumb breadth), digit 1 length (thumb length), and hand length

from digitizer (hand length). Greiner (1991) provided definitions

and illustrations of the three measures. Table 1 summarizes the

subjects hand and finger sizes.

2.2. Experimental design

A within-subjects design was used in the experiment, in which

two within-subjects variables (touch key size and touch key loca-

tion) were manipulated. The touch key size factor had three levels

(square shape with 4 mm, 7 mm, and 10 mm wide). A pilot test was

conducted toselect thelevels of thetouch keysize, inwhicha total of

eight different touch key sizes ranging from 3 mm to 13 mm

(i.e. 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, 10 mm, and 13 mm)

were evaluated from a performance and user satisfaction stand-

points. The pilot test reported that the 3 mm touch keys tended to

provide poor usability, which seemedto be much lower than that of

the4 mmtouchkey. On theother hand,the touchkey sizes of10 mm

and 13 mm seemed to provide the best usability and it was difficult

to find difference between them. Therefore, although the pilot test

did not provide statistically significant results, 4 mm and 10 mmwere selected for a further experimentation as small and large sizes

applicable to mobile phones. The 7 mm touch key size was also

selected since it is the mid-point between 4 mm and 10 mm.

The touch key location factor had 25 levels. Each location was

one of the center points of 25 rectangular areas that had the same

width and height (that is,one-fifth of a touch screen width andone-

fifth of a touch screen height, respectively). In the experiment, the

center point of a square touch key was located at one of 25 touch

key locations. Fig.1 shows the 25 touch key locations and their IDs.

Fig. 2 presents an example of experimental targets with the touch

key size of 10 mm and the touch key location of 9.

2.3. Dependent measures

Two types of dependent measures (pressing performance and

subjective satisfaction) were collected in the experiment. The

Table 1

Summary of the subjects hand and finger sizes.

Dimension Mean

(mm)

Standard

deviation

(mm)

Maximum

(mm)

Minimum

(mm)

Digit 1 interphalangeal

joint breadth

(thumb breadth)

20.5 1.2 23.4 17.7

Digit 1 length

(thumb length)

58.4 4.4 68.3 49.2

Hand length

from digitizer

(hand length)

182.8 7.9 194.8 161.4

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Fig. 1. Touch key locations used in the experiment.

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The main experiment consisted of three blocks, in which the

subjects were asked to press targets on a touch screen as fast as

possible. Each block had 25 experimental conditions (i.e. 25

different touch key locations with the same touch key size). For

each experimental condition, the two-state experimental task was

repeated 10 times. That is, a total of 250 tasks were performed foreach block. After completing each block, the subjects were asked to

rate the pressing convenience for 25 touch key locations. The

presentation order of the three blockswas determined by the Latin-

square balancing technique.

3. Results

Four measures (first transition time, task completion time,

number of errors, and pressing convenience) were collected for the

seventy-five experimental conditions (3 sizes by 25 locations).

Table 2 summarizes the means and standard deviations for the 75

experimental conditions.

3.1. First transition time

The first transition time was statistically analyzed using the two-

way ANOVA test. The results showed that all the main effects (touch

key size (F(2, 58)24.7, p< 0.01), touch key location (F(24,

696)34.0, p< 0.01)) and the interaction between them (F(48,

1392)5.7,p< 0.01) were significantat the0.05 significancelevel. As

post-hoc analyses, the StudentNewmanKeuls (SNK) tests were

conducted on the significant main effects (i.e. the touch key size and

the touch key location). In addition, the simple effect test was con-

ducted on the significant interaction effect.

The SNK tests revealed that pressing touch keys with the size of

4 mm took longer first transition time than pressing touch keys with

the other sizes (i.e. 7 mm and 10 mm). The difference of the first

transition time between 7 mm and 10 mm was not statisticallysignificantat the0.05 significancelevel.Fig.4 presents mean valuesof

the first transition time.

Two groups of the touch key locations, good regions and poor

regions, were identified by the SNK test on the touch key loca-

tions. Good regions provided good usability in terms of eachusability measure, while poor regions provided poor usability.

Good regions provided statistically better usability than poor

regions at the 0.05 significance level. Then, a series of SNK tests

were performed with partial data separated by the touch key

sizes, because the simple effect of the touch key location was

significant for each level of the touch key size (for 4 mm key size,

F(24,696)22.5, p< 0.01; for 7 mm key size, F(24,696)29.5,

p< 0.01; for 10 mm key size, F(24,696)13.2, p< 0.01). Fig. 5

illustrates good and poor regions identified by the four SNK tests,

i.e. one for considering all touch key sizes together and three for

each touch key size. Fig. 5 shows that center regions tend to

require shorter first transition time than edge regions. Specifically,

the touch key locations of 8, 13, 14, 18 and 19 (good regions) take

the shortest first transition time from the SNK tests, while edge

regions including IDs of 1, 2, 16, 21, and 25 (bad regions) require

the longest first transition time.

Table 2

Means and standard deviations for each experimental condition (Numbers in parenthesis are standard deviations).

IDa First transition time (ms) Task completion (ms) Number of errors Pressing convenience (points)

Size: 4 mm Size: 7 mm Size: 10 mm Size: 4 mm Size: 7 mm Size: 10 mm Size: 4 mm Size: 7 mm Size: 10 mm Size: 4 mm Size: 7 mm Size: 10 mm

1 1209.3 (180.7) 1077.3 (177.5) 1047.0 (177.9) 1559.1 (396.8) 1186.3 (203.1) 1102.2 (175.5) 0.53 (0.39) 0.21 (0.18) 0.11 (0.16) 2.2 (1.6) 3.3 (2.0) 3.8 (2.2)

2 1176.0 (144.5) 1065.1 (109.7) 977.5 (176.0) 1616.1 (470.6) 1166.6 (163.2) 991.1 (175.7) 0.69 (0.54) 0.20 (0.20) 0.03 (0.04) 3.3 (2.2) 4.7 (1.8) 5.3 (2.2)

3 1061.2 (103.1) 955.5 (115.3) 943.2 (137.0) 1388.7 (293.4) 1035.3 (147.2) 965.2 (138.2) 0.58 (0.52) 0.15 (0.13) 0.05 (0.06) 4.5 (2.2) 5.7 (1.9) 6.2 (2.0)

4 1036.7 (106.2) 915.0(110.1) 947.6 (170.2) 1663.4 (890.6) 1034.2 (140.8) 984.9 (195.3) 0.89 (0.77) 0.23 (0.20) 0.07 (0.12) 4.0 (1.8) 5.6 (1.8) 6.1 (1.7)

5 1158.5 (157.0) 917.5 (100.4) 896.3 (102.5) 1711.4 (800.3) 972.6 (86.9) 917.3 (92.1) 0.78 (1.00) 0.12 (0.11) 0.05 (0.08) 2.9 (1.5) 4.4 (1.9) 5.4 (1.9)

6 1137.0 (163.9) 975.3 (102.0) 938.1 (135.1) 1517.2 (387.9) 1047.0 (136.8) 966.5 (166.0) 0.51 (0.39) 0.14 (0.20) 0.05 (0.11) 3.2 (2.0) 4.7 (1.9) 5.2 (2.0)

7 1033.4 (137.7) 922.2 (108.3) 910.4 (126.0) 1301.9 (334.5) 1011.2 (140.3) 938.1 (133.9) 0.43 (0.37) 0.17 (0.16) 0.05 (0.09) 5.6 (2.1) 6.9 (1.8) 7.3 (1.8)8 989.7 (127.4) 831.2 (60.4) 840.4 (84.8) 1335.0 (384.6) 914.0 (119.6) 861.3 (92.9) 0.54 (0.32) 0.16 (0.20) 0.05 (0.07) 6.0 (1.8) 7.5 (1.6) 8.1 (1.2)

9 951.6 (96.4) 845.0 (74.5) 919.9 (139.6) 1517.3 (567.5) 936.4 (149.5) 959.1 (167.4) 0.87 (0.60) 0.17 (0.18) 0.06 (0.08) 5.6 (1.9) 7.1 (1.6) 7.8 (1.4)

10 1012.7 (111.0) 879.9 (86.0) 853.5 (91.7) 1314.7 (432.6) 935.9 (98.0) 877.6 (99.4) 0.53 (0.61) 0.12 (0.14) 0.05 (0.08) 3.2 (1.4) 4.9 (1.7) 6.0 (1.8)

11 1060.9 (161.1) 925.1 (120.0) 925.5 (161.7) 1259.3 (233.5) 981.7 (115.3) 937.7 (167.0) 0.32 (0.25) 0.11 (0.13) 0.03 (0.08) 4.2 (2.0) 5.4 (1.6) 5.8 (1.7)

12 994.0 (96.9) 864.3 (107.0) 874.5 (101.1) 1281.8 (351.7) 937.4 (126.1) 904.2 (116.9) 0.47 (0.41) 0.16 (0.15) 0.06 (0.07) 6.3 (1.9) 7.6 (1.3) 7.8 (1.4)

13 955.8 (101.9) 828.6 (86.9) 816.3 (103.9) 1377.0 (428.6) 912.3 (116.0) 856.2 (125.8) 0.61 (0.43) 0.17 (0.19) 0.05 (0.07) 6.8 (1.6) 8.3 (1.2) 8.6 (0.9)

14 913.9 (80.2) 812.4 (75.0) 860.0 (123.4) 1484.7 (815.5) 918.0 (89.0) 947.3 (235.8) 1.00 (1.12) 0.22 (0.19) 0.15 (0.25) 6.0 (1.6) 7.6 (1.5) 8.1 (1.2)

15 1098.5 (137.0) 920.3 (102.2) 876.0 (81.4) 1441.4 (367.6) 994.3 (121.8) 920.4 (117.9) 0.55 (0.39) 0.13 (0.14) 0.09 (0.12) 2.9 (1.3) 4.6 (1.9) 5.6 (2.2)

16 1045.9 (115.8) 946.5 (121.7) 967.8 (187.4) 1265.0 (210.9) 994.1 (180.5) 990.0 (241.6) 0.37 (0.29) 0.08 (0.11) 0.04 (0.12) 3.4 (1.7) 4.9 (1.8) 5.4 (1.8)

17 1018.8 (121.0) 905.7 (130.8) 875.8 (121.4) 1380.5 (427.0) 1021.9 (175.3) 906.0 (152.7) 0.58 (0.48) 0.20 (0.17) 0.07 (0.15) 5.9 (2.0) 6.9 (1.4) 7.4 (1.4)

18 970.2 (99.1) 853.5 (93.4) 845.4 (93.3) 1253.0 (341.1) 911.0 (103.7) 857.3 (99.2) 0.44 (0.36) 0.11 (0.13) 0.02 (0.04) 6.0 (1.8) 7.4 (1.3) 8.0 (1.0)

19 1012.7 (114.0) 872.2 (85.1) 864.2 (133.5) 1475.6 (479.7) 965.9 (138.2) 922.3 (195.0) 0.73 (0.63) 0.18 (0.23) 0.10 (0.17) 5.0 (1.9) 6.7 (1.8) 7.3 (1.5)

20 1136.5 (165.4) 963.3 (138.2) 913.2 (98.8) 1583.6 (500.7) 1104.5 (168.7) 981.6 (160.5) 0.66 (0.59) 0.26 (0.19) 0.11 (0.17) 2.1 (1.0) 4.2 (1.8) 5.1 (2.2)

21 1173.7 (218.4) 1038.2 (216.3) 984.5 (157.9) 1553.6 (488.6) 1169.9 (412.9) 1061.6 (206.6) 0.59 (0.51) 0.24 (0.49) 0.15 (0.19) 2.4 (1.5) 3.8 (1.8) 4.5 (1.9)

22 1105.6 (178.2) 951.0 (127.6) 920.9 (110.1) 1612.9 (624.2) 1144.2 (299.8) 1019.4 (292.0) 0.72 (0.60) 0.35 (0.36) 0.17 (0.31) 3.2 (1.3) 4.6 (1.6) 5.6 (1.9)

23 999.8 (98.2) 895.2 (73.4) 888.0 (114.7) 1350.0 (369.2) 1003.8 (139.7) 958.8 (189.2) 0.60 (0.51) 0.23 (0.26) 0.12 (0.16) 3.7 (1.6) 5.1 (1.6) 6.1 (1.9)

24 1040.3 (113.7) 911.2 (106.6) 876.9 (84.8) 1587.4 (519.2) 1086.8 (237.3) 916.3 (98.7) 0.89 (0.72) 0.33 (0.33) 0.09 (0.10) 2.7 (1.6) 4.2 (1.6) 5.6 (2.0)

25 1089.3 (122.5) 963.9 (110.7) 948.5 (145.2) 1551.1 (464.3) 1117.2 (185.4) 1044.4 (194.6) 0.74 (0.57) 0.29 (0.25) 0.16 (0.19) 1.7 (1.1) 3.0 (2.0) 4.3 (2.4)

a IDs of touch key locations.

0

200

400

600

800

1000

1200

1400

4 mm 7 mm 10 mm

Touch key size

A B B

1055.3

921.4 908.5

Meanfirsttransitiontime(msec)

Fig. 4. Mean first transition time (ms) of the touch key sizes. The same letter indicates

that those conditions were not significantly different from each other.

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3.2. Task completion time

The same statistical techniques were used to analyze the task

completion time. The ANOVA test showed that the touch key size

(F(2,58)88.3, p< 0.01), the touch key location (F(24,696)6.4,

p< 0.01), and the interaction between them (F(48,1392)2.2,

p< 0.01) influenced the taskcompletion timeat the 0.05significance

level.

Similartothe results of thefirsttransitiontime, thetouch keysize

of 4 mm required the longest task completion time, while the 7 mm

and 10 mm sizes were not different to each other at the 0.05

significance level. Fig. 6 presents mean task completion time.The SNK test on the touch key location, also, provided good and

poor regions considering all touch key sizes together. The simple

effect tests showed that the task completion time was significantly

influenced by the touch key locations when the touch key size was

fixed at each factor level (for 4 mm, F(24,696)3.0, p< 0.01; for

7 mm, F(24, 696) 10.1, p< 0.01; for 10 mm, F(24,696) 7.7,

p< 0.01). Then, good and poor regions for each touch key size were

identified by the SNK tests at the 0.05 significance level. Fig. 7

illustrates good and poor regions under the four conditions of the

touch key size. For the task completion time, good and poor regions

are different according to the touch key size. Left areas on a touch

screen tend to be good regions for the smallest touch keys, while

center and right areas tend to be good regions for the other touch

key sizes (i.e. the touch key size of 7 mm and 10 mm). With respectto poor regions, the touch key location of 5 is a poor region for the

4 mm touch key size, while upper leftmost locations (IDsof 1 and 2)

and the lowermost locations (IDs of 21, 22, and 25) are poor regions

for the other touch key sizes.

3.3. Number of errors

The one-way ANOVA on ranks, one of the non-parametric statis-

tical techniques, was applied to the number of errors (Hesel and

Hirsch, 2002). It required transformation of the collected data to the

rank ones before applying the ANOVA. Then, the SNK tests, as post-

hoc analyses, were performed on the significant effects (Aref,1995).

0

400

800

1200

1600

2000

2400

4 mm 7 mm 10 mm

Touch key size

A B B

1455.3

1020.1 951.5

Meantaskcompletiontime(msec)

Fig. 6. Mean task completion time (ms) according to the touch key size. The same

letter indicates that those conditions were not significantly different from each other.

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For all touch key sizes

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Touch key size: 4mm

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Touch key size: 7mm

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Touch key size: 10mm

Fig. 5. Good and poor regions in terms of the first transition time. The dark areas and white areas represent good and poor regions, respectively. The gray areas represent regions

that provide an average level of the first transition time.

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Touch key size: 4mm Touch key size: 7mm Touch key size: 10mmFor all touch key sizes

Fig. 7. Good and poor regions in terms of the task completion time. The dark areas and white areas represent good and poor regions, respectively. The gray areas represent regions

that provide an average level of the task completion time.

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The touch key size significantly affected the number of errors

(F(2,58) 871.0, p

touch key design

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