Does Size Matter? Investigating the impact of mobile phone screen size on users’ perceived usability,

effectiveness and efficiencyDimitrios Raptis, Nikolaos Tselios, Jesper Kjeldskov, Mikael Skov

PhD Fellow, Computer Science Department, Aalborg University

our idea

D i m i t r i o s R a p t i s , h t t p : / / p e o p l e . c s . a a u . d k / ~ r ap t i s

does the size of the screen affect users’ interaction with a mobile device when they use a mobile application?

is this effect significant?

if yes is this effect consistent for various screen sizes?

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research hypotheses

goal: measure the effect of a mobile phone’s screen size on users’ perceived usability, efficiency and effectiveness

D i m i t r i o s R a p t i s , h t t p : / / p e o p l e . c s . a a u . d k / ~ r ap t i s

H 1: the mobile phone’s screen size will have an effect on participants’ perceived usability (a larger screen will increase the perceived usability ratings).

H 2: the mobile phone’s screen size will have an effect on participants’ task completion times (a larger screen will increase efficiency).

H 3: the mobile phone’s screen size will have an effect on participants’ task completion rates (a larger screen will increase effectiveness).

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parameters can that affect (perceived) usability

D i m i t r i o s R a p t i s , h t t p : / / p e o p l e . c s . a a u . d k / ~ r ap t i s

attractiveness

effectiveness

efficiency

prior experience

brand

prior usage perceptions

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device/application

device and application selection

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3 devices of the same brand, with identical visual design

IMDB 2.3.1, an information seeking application

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Model Size (mm) Scr.Size

Resolution

Galaxy Ace 112.4x59.9x11.5 3.5in 320*480

GalaxyNote 145.8x66.1x8.5 5.3in 800*1280

Galaxy SII 125.3x66.1x8.5 4.3in 480*800

D i m i t r i o s R a p t i s , h t t p : / / p eo p l e . c s . a a u . d k / ~ r a p t i s

pre-experiment actions

aim: to provide an as similar as possible interface on three devices with similar hardware performance

we installed the same Android versions, same launcher, same theme, same keyboard.

we locked the screen orientation

we underclocked the CPUs (800Mhz)

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experiment

D i m i t r i o s R a p t i s , h t t p : / / p e o p l e . c s . a a u . d k / ~ r ap t i s

60 volunteers (48 male, 12 female), aged 19-30 (M=23.48, SD=2,34)

divided in three groups (20 per group)

performed 5 specific tasks of varying difficulty and scrolling needs

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D i m i t r i o s R a p t i s , h t t p : / / p eo p l e . c s . a a u . d k / ~ r a p t i s

used questionnaires/extracted variables demographics questionnaire

attractiveness questionnaire

evaluation questionnaire (SUS +)

task completion times and rates

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extracted variables:1)SUS Score2)adjective rating 3)owned screen size4)prior experience with application 5)prior experience with Android

6)will to buy the used device 7)favorite screen size8)favorite brand9)attractiveness

the effect of screen size on perceived usability (H1)

ANCOVA: screen size (independent), SUS Score (dependent), prior experience (covariate 1), desire for the device (covariate 2)

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two components were extracted (principal component analysis):

prior experience (prior experience with Android and prior experience with the application)

desire for the device (attractiveness, will to buy the used device, favorite screen size and favorite brand)

results:

no main effect of screen size (F(2,55)=.978, p=.383)

a significant effect of prior experience (F(1,55)=5.890, p=.019*)

a significant effect of desire for the device (F(1,55)=4.910, p=.031*)

H 1 was rejected

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the effect of screen size on efficiency (H2)

3x5 mixed design ANOVA (five levels: Tasks 1-5)

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a significant main effect of screen size on participants’ efficiency was unveiled (F(2.37,57)=10.3, p<.001**)

pairwise comparisons:

Galaxy Ace (3.5in) - Galaxy Note (5.3in), p=.007**

Galaxy SII (4.3in) - Galaxy Ace, p=.107

Galaxy SII - Galaxy Note, p=.253

finding:a significant gain on efficiency occurs when the screen size increases more than 4.3in

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the effect of screen size on efficiency (H2)

One way between subjects ANOVA

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we observed a significant effect of screen size on efficiency for:

Task 3, F(2, 57)= 6.016, p=.004**

Task 5, F(2, 57)= 4.559, p=.015*

finding:not all information seeking tasks benefit from a larger display, but only the tasks that are not easy and require a significant amount of scrolling.

H 2 was confirmed11 of 17

the effect of screen size on effectiveness (H3)

CHI Square tests per task (completion rate)

D i m i t r i o s R a p t i s , h t t p : / / p e o p l e . c s . a a u . d k / ~ r ap t i s

H 3 was rejected as there was not observed any significant effect of screen size on effectiveness

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other important observed effects a significant high correlation between adjective

rating and SUS score (Pearson’s r=0.679, p<.001**)

a significant modest correlation between total task completion time and SUS score (Pearson’s r=-0.357, p=.005**)

a significant difference between the participants’ SUS rating with prior experience (44 participants) and without experience (16 participants), two tailed T-test, t=2.141, p=.04*

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findings

everyday usage

D i m i t r i o s R a p t i s , h t t p : / / p e o p l e . c s . a a u . d k / ~ r ap t i s

mobile users:

when they interact with a device and perform information seeking tasks, they will be more efficient using a device with a screen larger than 4.3in.

designers:

they should consider alternative ways to minimize scrolling

in tailored applications for specific contexts they should consider to select devices with screens larger than 4.3in

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findings

practitioners/researchers

D i m i t r i o s R a p t i s , h t t p : / / p e o p l e . c s . a a u . d k / ~ r ap t i s

researchers and practitioners that measure perceived usability through SUS will not observe any differences if they evaluate an application on devices with different screen sizes.

researchers and practitioners that assess usability through usability metrics, such as effectiveness and efficiency, they will observe that larger screens will lead to higher efficiency.

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future work

D i m i t r i o s R a p t i s , h t t p : / / p e o p l e . c s . a a u . d k / ~ r ap t i s

study the effect of screen size on an expanded set of tasks, such as map navigation, video browsing, etc.

expand the devices in order to measure if and when the identified positive effect on efficiency stops/declines.

investigate in detail if the participants of a mobile usability laboratory evaluation are influenced regarding their SUS scores, when they are asked to evaluate an application using a mobile device that has the same screen size as the one they own.

study the screen size effect over time, in real world conditions as the familiarity with a device increases.

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D i m i t r i o s R a p t i s , h t t p : / / p e o p l e . c s . a a u . d k / ~ r ap t i s

Thank you!!

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D i m i t r i o s R a p t i s , h t t p : / / p eo p l e . c s . a a u . d k / ~ r a p t i s

device familiarity

a significant effect from a two-way ANOVA between favorite/owned screen size (F(3, 48) = 5.887, p = .002**) and screen size F(2, 48) = 6.525, p = .003**) on SUS score, (F(6, 48) = 3.663, p = .004**)

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limitations

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the test application’s interface was identical, but not exactly the same on all three devices

the Galaxy SII and Note had the same AMOLED screen, but not the ACE

we did control for screen brightness but not accurately

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