Trees and Nests: A Comparison between Two Hierarchical Metaphors in Educational Applications

doi:10.4236/ce.2012.31021

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Creative Education

2012. Vol.3, No.1, 126-133

Published Online February 2012 in SciRes (http://www.SciRP.org/journal/ce) http://dx.doi.org/10.4236/ce.2012.31021

126

Trees and Nests: A Comparison between Two Hierarchical

Metaphors in Educational Applications

Adi Katz

Department of I nd us trial Engineering and Management, Shamoon College of Engineering, Ashdod, Israel

Email: adis@sce.ac.il

Received December 2nd, 2011; revised January 6th, 2012; accepted January 17th, 2012

The aim is to determine whether one of two hierarchical metaphors, the tree (parent-child) or the nested

(object-container), is more suitable for designing educational interfaces for children. To cope with this is-

sue an experimental educational application was designed with a prototype for each hierarchical metaphor.

The application was evaluated in a laboratory experiment, where children participants interacted with the

prototypes to find answers for questions that require searching for information. Task performance was

measured in terms of effectiveness, efficiency and subjective aspects such as user perception of ease of

use and user preference. The nested (object-container) metaphor was found to be preferred by users and

superior in several objective parameters of performance efficiency, but no significant differences were

found in the perceived ease of use and in the performance effectiveness. Implications for designing edu-

cational applications are discussed.

Keywords: Human Computer Interactions; Metaphoric Interfaces; Hierarchical Information Organization;

Educational Applications; Usability; Children Interfaces Design

Introduction

Today, more than ever, children are exposed to computers in

their early years of development and have easy access to com-

puters and the Internet. Children use the Internet for school-

work, playing games and communicating with each other, and

are typically involved in activities that require searching and

browsing for information (Hutchinson et al., 2006). Creating

interfaces for young children presents particular challenges, and

the designers of such applications must take into consideration

that children are different than adults in the way they think and

learn. An important requirement for an effective learning proc-

ess is to foster a positive attitude and affect that nourish moti-

vation, arouse curiosity, engage creativity, and turn the brain

into an effective learning organism (Norman, 2004). It is most

important that applications for children are provided with suit-

able interfaces that make both their learning process and their

interaction with the applicatio n easy and enjoyable.

A successful user interface paradigm is to design the hu-

man-computer interaction based on metaphors already familiar

from real life objects, actions and situations. A metaphoric

system simply replicates the real life objects and portrays them

in the computerized environment, so that the user can intui-

tively manipulate objects and work in a familiar manner. A

suitable metaphor enables the user to function effectively in the

new system, and helps overcome cognitive limitations in com-

plex tasks (Te’eni et al., 2007). One of the implications of the

cognitive load theory (CLT) in the area of instructional design

is that the layout should be intuitive, so that the mind’s capacity

for processing information will not be overloaded and user

activities will be focused on the concepts to be learned (Sorden,

2005). Instead of learning the system and its features, meta-

phorical designs free some of the users’ limited cognitive re-

courses so they can be fully engaged and devoted to the data

and the activities at hand. When cognitive resources are unnec-

essarily devoted to mental activities such as retrieving from

memory and learning new objects and actions, it may be at the

expense of performance. Metaphors have a particularly impor-

tant function in interfaces for young children (Ellis & Blashki,

2001; Gilutz & Black, 2010), are highly preferred by them and

can induce curiosity and pleasure (De Angeli et al., 2006). A

useful metaphor must be suitable for the user population (Katz

& Vaserman, 2009), and therefore, when designing for young

users, the chosen metaphor must be borrowed from their eve-

ryday environment or conceptual world.

In order to cope with the overwhelming diversity of objects

and properties in the world, people mentally group objects,

treating them as instances of categories instead of as unique

individuals. Hierarchical organization is one type of a natural

evolution of a classification system in which people notice both

distinctions and similarities among objects, and organize cate-

gories into hierarchies in which more specific classes are in-

cluded in more general ones (Markman, 1989). In other words,

many natural categories are hierarchically organized into nested

class-inclusion relations, where some classes are super-ordinate

or subordinate to others. For example: organisms/plants/flow-

ers/Composite family/daisies/erbera daisies. Collins and Quil-

lian’s (1969) Hierarchical Network Model of semantic memory

states that concepts are stored and represented as nodes within a

hierarchical structure in our long-term memory (LTM), with

meaningful associations between concepts. The “Is A” link is

the most common link in this semantic network model.

Browsing for information is a methodical activity with a spe-

cific goal, which requires progressive filtering of results based

on visual scanning and searching. Since hierarchical organiza-

tion stems from greater accumulation of knowledge and ex-

perience (Markman, 1989), children have a difficulty dealing

with hierarchies when searching and browsing. Children do not

always navigate efficiently between categories, they may not

A. KATZ

think hierarchically, and may have a difficulty understanding

abstract, top-level categories (Hutchinson et al., 2006). Meta-

phors may be a useful design paradigm to make it easier for

children to cope with browsing through hierarchical levels of

information. A metaphor based on a hierarchical organization

that children are already familiar with, actually utilizes knowl-

edge that has already been accumulated and this, as previously

described, decreases the cognitive overload. Clearly, the chosen

metaphor should also be appropriate for presenting the needed

relationships between categories. For example, an application

teaching organism hierarchies should present “Is-a” relation-

ships (A grizzly bear Is-a bear and a bear Is-a land mammal).

A well-known system for organization and management of

files and folders on our computing devices embodies two hier-

archical metaphoric-based interfaces; one is the file manager

interface, which reflects the tree metaphor, and the other is the

desktop interface, which is based on a nested object-container

metaphor. The tree metaphor layout that originates from the

very first instantiations of computer desktop environments, has

come out so pervasive that is firmly embedded into our intui-

tive ways of dealing with “items”, be it images, videos or gen-

erally speaking any type of information that can be defined as

“files” (Villegas, 2006). Users navigate by clicking on little

structural elements (plus and minus symbols) that open and

close various directories.

The nested (object-container) metaphor for hierarchical or-

ganization expresses the relationship of “object-within-object”

and allows recursion to be visualized in an intuitively nested

“Russian doll” fashion. The nested Russian doll is widely used

in recursive problem solving and in programming and mathe-

matics education to explain recursive models (Schiemenz,

2002). The most obvious example of the nested doll principle in

computers is Microsoft Windows, where “windows” is simply

windows inside windows inside windows.

The current research follows a previous study that compared

the tree and the object-container metaphors in the forms of a

family tree versus a cabinet of drawers—two alternative meta-

phoric designs to convey hierarchical learning of material. In

the previous study no superiority of one metaphor over the

other in the design of children’s interfaces was found (Katz and

Vaserman, 2009). Despite these results, we were still certain

that a cabinet metaphor was more suitable for children since it

inherits affordances that are the actions of organizing their be-

longings (toys, books, candy, etc.) in their natural environment

at playrooms in their homes and kindergartens. Therefore the

expectation was that this metaphor would be more intuitive and

easy to operate, more usable and more preferable for young

children than the tree metaphor. We believe that the previous

results were obtained due to specific design elements of the

metaphoric interface that caused an unwanted difference in

usability between the tree and the cabinet designs. In addition it

should be noted that the previous research was based on a small

sample of only 10 subjects. In this research we decided to re-

design the metaphors and test our hypotheses on a larger sam-

ple of subjects.

Problem Definition and Solution

Young children need encouragement, support and help in

learning about the world and developing cognitively and emo-

tionally. Pre-school children are at a very important stage of

development in life, in which they are exposed to activities of

the early learning of reading and writing. At this phase, most

children are constantly involved in confident exploring behav-

iors, interacting with their environment in an active way and

expanding their cognitive abilities on the basis of their own

activities. We believe that educational tools such as learning

applications for children should serve as a vehicle to promote

learning, curiosity, exploratory behavior and independence in

the learning process, and serve to flourish their knowledge. In

addition, successive interactions and positive learning experi-

ences at this stage in childhood can develop a high level of self

confidence, esteem and efficacy. A sense of self efficacy has a

positive influence on the success of the learning process, be-

cause previous successful and positive experiences develop

expectations to succeed in similar tasks (Bandura, 1997). Self

efficacy developing while interacting with an application used

for learning gives children the confidence to continue learning

with the same tool, and motivates them to explore additional

tools and features.

Creating interfaces for young children presents particular

challenges. Children have different cognitive capabilities than

adults; they think and learn differently. The variable of age en-

compasses many critical developmental differences between

childrens’ and adults’ ability to interact with technology, and

their unique characteristics play an important role in creating a

successful user experience for them. This is why usability test-

ing techniques should be applied with a user centered design

(UCD) approach, particularly when designing for children to

meet their capabilities, needs and expectations (Gilutz & Black,

2010).

The objectives for this research were to:

1) Design and implement the features of two hierarchical re-

cursive metaphor prototypes for a computer program that edu-

cates children about animals: the tree and the nested metaphors.

2) Compare the two prototypes by conducting an experiment

with pre-defined searching tasks, using subjective quantitative

measures of the perceived ease of use (perceived usability) and

user preference, as well as objective and quantitative measures

of effective and efficient performance. In addition, gather qua-

litative data by directly observing the child-application interact-

tion.

The Experiment

The two hierarchical metaphors, tree and nested, were tested

in a controlled experiment, in which children interacted with

two metaphoric-based educational systems to answer questions

regarding animals. The systems were aimed at the age group

between four and six since most children at this age range are

still unfamiliar with the file manager (tree) and nested-windows

previously mentioned, and are therefore not influenced by con-

ventional tree and nested metaphoric designs. This enabled

testing the suitability of the metaphors without excessive noise.

The metaphor design, tree versus closet, was manipulated to

result in a natural tree design for the tree metaphor and a play-

room closet design for the nested metaphor.

The Dependent Variables

The subjects’ performance was observed in terms of efficiency

and effectiveness, user usability perceptions, and preference.

Table 1 presents the variables and how they were measured.

A. KATZ

Table 1.

The dependent varia bles and their measures.

Dependent variables Measure

Performance

 Effectiveness Overall number of correct answers

 Efficiency

 Overall task time

 Total number of clicks

 Total number of false clicks

Perceived ease of use

(usability) “How easy was it to search for answers in the

application?” (4-point Likert scale)

Preference “Which application wo uld you prefer having a t

home: the tree or the closet?”

The Independent and the Control Variables

The independent variable was the metaphoric design of the

interface: tree versus closet. The two designs are described in

detail in Section “Apparatus—Metaphoric Interface Design”.

The controlled variables were the participant, the tasks and

the order of task performance. The participants were thirty kin-

dergarten children (twelve girls and eighteen boys), all within

the target age group of four to six years old. The participants

were classified as novice users in terms of Shneiderman’s ge-

neric classification of users (Shneiderman, 1987) since they did

not possess significant semantic knowledge of objects and ac-

tions in computers such as the hierarchical organization of files

and folders, and lacked the syntactic knowledge of the meta-

phoric applications designed for this research. All participants

interacted with both metaphoric educational applications at the

kindergarten.

Each child had to carry out a series of search tasks to enable

answering a set of questions, such as: “what is produced from

tuna fish liver?”, and “how does a gorilla react to threat?” Overall

there were fourteen questions regarding animals, with seven

different tasks for each application. Two different but structur-

ally and cognitively similar sets of tasks were created, with cor-

responding questions in each application. For example, a ques-

tion appearing in the tree metaphor was: “what can be made of

goose-feathers” and the corresponding question in the closet

metaphor was: “what is produced from tuna fish liver”.

In order to avoid possible bias due to the order of exposure of

the subjects to each metaphor, the children were randomly di-

vided into two groups of 15 children each, so that one group

interacted with the tree prototype first and continued with the

closet, and the second interacted in the opposite order (hereafter,

Tf and Cf will be used as abbreviations for Tree first and Closet

first, respectively). After having conducted seven search tasks

in the first application, the applications were switched to per-

form the additional seven search tasks in the second applica-

tion.

Pilot Test

After having implemented the two prototypes to meet the de-

sign criteria of the two hierarchical metaphors, both applica-

tions were pilot tested on six children as representatives of the

potential users, to obtain feedback regarding the design, to

identify potential usability problems, and to make sure that the

experiment was not too difficult. The pilot participants were

three girls and three boys between the ages 4.5 - 6. Each used

both prototypes where three started with the tree metaphor and

the other three started with the closet metaphor. The pilot was

conducted so as to follow the user centered design (UCD) ap-

proach, to ensure the prototypes were compatible with the end-

users’ characteristics , such as cognitive development, cognitive

limitations, preferences and other factors that may have af-

fected the understandability and usability of the applications.

The pilot participants’ interaction with the applications was

observed and the design was adjusted accordingly. UCD is

extremely important when designing for children, and is widely

used in the HCI literature in various variations such as coopera-

tive inquiry and participatory and informant design (Baek &

Lee, 2003; Druin, 2005; Mazzone et al., 2008). The pilot re-

sulted in some improvements in design features that were not

easy or intuitive or caused confusion, and in several experi-

mental changes.

Apparatus—Metaphoric Interface Design

The research was conducted in Israel, and therefore the in-

terfaces’ language was Hebrew. The two educational programs

were identical in the type and amount of information, in the

hierarchical categorization and levels (with “Is-a” relations be-

tween categories), and in icons and labels representing object

categories.

Figure 1 presents the opening screens of the hierarchical met a-

phors: the tree metaphor on the left side, and the closet meta-

phor on the right. Figure 2 presents two compatible screens of

the tree and the closet metaphors. The hierarchical classes that

are displayed in both screens represent the following hierarchi-

cal levels: Mammals/Land Mammals/Ungulates/Horse; Deer;

Cow; Donkey.

As aforementioned, specific design elements of the metaphoric

interface have caused an unwanted difference in usability be-

tween the prototypes in the previous study comparing between

the tree and cabinet metaphors (Katz & Vaserman, 2009). The

main problem was that the cabinet metaphor was inferior in

usability characteristics not directly derived from the metaphor,

such as a relative lack of information regarding the hierarchy in

the visualization of hierarchical organization, and the use of a

confusing navigational aid. Drawing conclusions from that

research, great emphasis was placed on a unified transitioning

between various hierarchical levels in both prototypes. This

goal was accomplished by requiring an equal number of clicks

to move from one level of the hierarchy to another, and design-

ing each prototype to manifest the complete hierarchical path

for each choice of the user.

Figure 1.

The opening screens of each metaphoric interface.

128

A. KATZ

In the tree metaphor (Figure 2(a)), the four main (general)

categories were displayed at the bottom part of the screen,

closest to the tree’s roots, and the highest branch displayed the

most specific (leaf level) categories. This is obviously a natural

way to present a physical tree that has a “bottom-up” develop-

ment growing upwards from the roots in the ground. An alter-

native metaphoric design that utilizes the tree metaphor is the

family tree (Katz & Vaserman, 2009; Park & Park, 2010), but it

is typically drawn “top-down”, presenting ancestors above their

descendants. Note that both versions of the tree metaphor con-

ceptually convey a “top-down” relation, in which categories

start with the most general class, until reaching the lowest level

of elementary subclass. The claim is that while adults distin-

guish between “top-down” and “bottom-up” in many areas that

differentiate levels of abstraction, children are not aware of this

“vertical flip” of real-life trees and hierarchical category de-

velopment and therefore will find it easier to handle a hierar-

chical tree metaphor organizing information in a manner that

imitates the natural growth of trees.

The tree metaphor endows the affordance of following paths

from the root, and from one branch to another. Paths are re-

vealed and concealed by unfolding and folding tree branches,

respectively, by clicking on categorical nodes. Clicking on a

node where the continuous branch seems hidden unfolds it

above that node (in other words, expands the tree upwards),

while clicking on an internal node folds the continuous branch

back by hiding it. When clicking on another node, at the same

or at an upper level in the hierarchy, the current node with an

open continuous branch folds back to the level of the clicked

node, and a new continuous branch unfolds at the clicked node.

The reason for folding a branch when the user wishes to unfold

another is to prevent a cognitive load that is likely to occur

when too much and irrelevant information is exposed. We used

navigation aids to highlight the hierarchical path, coloring the

chosen nodes (categories) light blue, while leaving the other

nodes (categories not on the path) in light green, and also by

presenting the branches connecting chosen nodes in sharp col-

ors, while the other branches in faded colors.

Following the idea of Katz and Vaserman (2009) of meta-

phoric designs for hierarchical organization for children, the

cabinet metaphor used in the previous study was redesigned to

a different variation of an object-container metaphor, which

was named the closet metaphor (Figure 2(b)). The closet meta-

phor matches the typical organization of objects (such as books

and toys) in children’s playrooms. While the Katz and Vaser-

man (2009) cabinet metaphor strictly followed the recursive

“Russian doll” fashion relations of doll within doll and so forth ,

by a cabinet of drawer within drawer and so forth, the newer

closet metaphor contains different objects at different levels of

the hierarchy, that can be placed one inside the other. The four

main categories (see Figure 2(b)) are displayed as the closet’s

four doors. Opening each door reveals its nested drawers that

contain boxes that contain animal cards representing the most

specific (leaf level) categories. We believe that the current

nested variation of the closet metaphor is more interesting and

fun than the previous, and is a better imitation of a playroom

closet that children are familiar with. Familiarity is an impor-

tant design factor that has a strong impact on children: the more

familiar is the interface metaphor, the better the comprehension

achieved (Gilutz & Black, 2010).

In Figure 2, one door—the mammals—is open to reveal two

drawers, land ma mmals and sea mammal s. The land mammals’

drawer is also open to reveal four boxes nested inside it: carni-

vores, ungulates, rodents and haplorhini. The ungulates’ box is

open and exposes four cards for the following animals: horse,

deer, cow and donkey. The closet-doors-drawers-boxes-cards

relationships are in a nested “Russian doll” fashion: the biggest

doll opens to reveal the next biggest doll, and so on until the

tiniest doll is revealed, nested in all dolls together. Opening

doors, drawers, and boxes (closed categories) is accomplished

by intuitively clicking on their handles, and closing these ob-

jects is accomplished by clicking on them again. Clicking on an

object that is closed (door, drawer or box) opens it and closes

the object that was open so far. For example, in the current path

presented in Figure 2, if one clicks on the sea mammal’s

drawer, it opens to reveal its boxes, and at the same time the

land mammal’s drawer closes. A click on the fish door reveals

its drawers, and causes the mammal door to close. The closet

metaphor endows a set of affordances that are extremely intui-

tive for young children: searching and revealing by opening and

closing doors, drawers, and boxes.

In both designs, clicking on last level nodes (leaf level cate-

gories) opens an information window displaying an educational

description of animal items, e.g. information about the deer or

donkey. The children were required to access the information

windows relevant to each task, to answer correctly. The number

of levels and categories at each level of the hierarchy is ad-

justed according to a recommendation referring to 5 ± 2 catego-

ries in children’s applications (Baek & Lee, 2003).

(a)

(b)

Figure 2.

screen shots of two compatible hierarchical paths for the two metaphors.

(a) Tree metaphor ; (b) Closet metaphor.

A. KATZ

130

Children need positive reinforcement and praise, which help

them to experiment successfully (Yahaya & Salam, 2009). To

motivate the participants’ involvement and arousal, we used

positive reinforcement in the form of a glass candy jar that is

gradually filled with a different candy for each task. Each time

a child chooses the right question; a candy appears at the upper

side of the screen and slowly falls down into the jar while

playing a funny sound of applause and cries of glee (“correct!”,

“well done!”). Animation and sound effects are positive ele-

ments in designing for children (Gilutz & Nielsen, 2007).

Whenever a wrong answer is chosen, a frowning “smiley” ap-

pears in an animated entrance with an audio encouraging the

child to try again. Audit output is crucial in interfaces designed

for young children, to over come the lack of a reading ability.

All text (displayed on buttons, information pages, instructions,

etc.) is presented with punctuations as customary in Hebrew

texts designated for young children, and is followed by a hu-

man voice that reads the text.

focus on gestures, facial cues and voice intonations. In addition

the “think aloud” technique was used, by encouraging the chil-

dren to speak out their thoughts, problems, indecisions and

impressions. The information gathered from these observations

added rich and useful information on the comprehensibility of

the designs and user experience, as later discussed.

Experimental Results

We used T tests for paired (dependent) samples to test per-

formance differences between the two prototypes: the tree ver-

sus the closet metaphor. The results are presented in Table 2. It

can be seen that there is significant difference in efficiency in

terms of overall task time, total number of false clicks, and total

number of clicks, in favor of the Closet metaphor application;

that is, it took considerably less time to answer the task ques-

tions, there were less false clicks and a smaller total number of

clicks when using the Closet application. However, there is no

significant difference in effectiveness in terms of the number of

correct answers. Note that almost all children answered all the

questions correctly with both applications.

Conducting the Experiment

Each of the children participants interacted separately with

both metaphors at a computer station in a quiet room at the

kindergarten, under our supervision. As said, 15 children were

randomly allocated to a group that interacted with the tree ap-

plication first and the closet application second, and the other

15 children interacted with the closet application first and the

tree application second. At the beginning of the interaction with

each application, the children received a short tutorial about it,

after which they practiced the use of the application by per-

forming two preliminary search tasks prior to the actual ex-

periment. The experiment itself was composed of fourteen tasks,

seven in each application. After having conducted seven search

tasks in the first application, the child performed the additional

seven search tasks in the second application.

A Wilcoxon signed ranks test was used to test the difference

in user perceptions of the ease of use of the metaphors (per-

ceived usability). The results indicate that there is no statistical

difference in the perception of ease of use between the tree

(Median = 2) and the closet (Median = 1) metaphors (Z =

–0.808, p = 0.419). For both prototypes, most users stated that

it was very easy or easy to use (26 and 28 for the tree and closet

prototypes, respectively) Figure 3 presents user rankings of

ease of use for each metaphor.

4- very ha rd3- hard2- easy1- very ea sy

Tr ee

Closet

During the experimental phase, the applications’ logs re-

corded the dependent variable measures of performance and at

the end of the interaction with each application, a question re-

flecting the child’s perceived ease of use of the application

appeared on the screen (“How easy was it to search for answers

in the application?”; the response was recorded on a 4 point

Likert scale). At the end of the experiment, after interacting

with both applications, a question regarding the child's applica-

tion preference appeared (“Which application would you prefer

to have at home: the tree or the closet?”).

During the experimental phase qualitative data was gathered

by direct observation of the child-application interaction, with a Figure 3.

Perceived ease of use of each metaphor.

Table 2.

Performance differences between the two interfaces.

Paired diffe rences

Performance Mean Std. D t df Sig. 2-tailed

Tree 11:43 3.11

overall task

time Closet 10:41 2.57 –2.742 29 .010

Tree 12.63 8.12

# of false

clicks Closet 9.13 6.14 –3.249 29 .003

Tree 42.77 9.66

Efficiency

total # of

clicks Closet 39.46 7.77 –2.452 29 .020

Tree 6.60 0.62

Effectiveness total # of

correct answers Closet 6.77 0.50 1.153 29 0.258

A. KATZ

Of 30 children participants, 12 (40%) preferred the tree pro-

totype and 18 (60%) preferred the closet. Six children said they

preferred the closet because it was easier to use for finding

information, 7 children said it was more fun to use, 3 said it

was prettier and 5 subjects said they preferred the closet be-

cause they had a similar closet at home (some gave more than

one explanation for their choice). Only one child said she pre-

ferred the tree because it was easier to use for finding informa-

tion, 2 children said it was more fun to use, and 2 said it was

prettier. Two subjects said they preferred the tree metaphor

because they liked trees and nature. These preference explana-

tions show that the metaphor evaluations were not only prag-

matic but also hedonic in nature. This point is elaborated in the

discussion.

The observations revealed that all children found interest in

both applications, and their visible behavior and verbal expres-

sions showed they were positively and happily engaged with

them. There were no signs of discomfort, frustration or dissat-

isfaction, on the contrary: the children were highly motivated to

find answers for the questions using both the tree and the closet

metaphors. The conclusion is that both metaphors were com-

prehensible and satisfying.

Discussion

We were interested in determining whether one of two hier-

archical metaphors, the tree or the closet was more suitable for

the design of children’s interfaces.

The results regarding interface efficiency, i.e., overall time to

perform tasks, number of false clicks, and total number of

clicks, show significant differences in favour of the closet

metaphor interface. The overall task completion time and total

number of clicks are related to Nielsen’s “efficiency” dimen-

sion of usability, and the number of false clicks is related to the

“few errors” dimension (1993). Therefore we can conclude that

in efficiency performance parameters (expressing the resources

consumed in the processes of achieving the task), the closet

metaphor is more usable than the tree metaphor. However, in

terms of performance effectiveness (i.e., the success of the task

in terms of the number of correct answers to the questions),

there was no difference between the two metaphoric designs.

Based on the results of T tests for independent samples to

test the differences between the performances of the groups in

the first and second exposure to the prototype, we can conclude

that the order of exposure to the prototypes does not matter.

Referring to the test of user perception of the interfaces’ ease

of use, the results do not show a significant difference: both

metaphorical interfaces were evaluated as easy or very easy to

use. We expected the closet metaphor to be perceived as more

usable for young children than the tree metaphor since it re-

sembles the way they organize their belongings in their play-

rooms and therefore would be more intuitive. Overall, the re-

sults show that both metaphors are perceived usable for kin-

dergarten children. Additional data, obtained from observations

of the children while they interacted with the applications,

showed that both metaphors were comprehensible, and there

was no significant difference in the difficulty expressed in the

action of transitioning between hierarchical levels.

The closet metaphor was designed to resemble any closet in

the natural environment of children’s playrooms. Choosing a

metaphor that matches between the system and the real world

of the user population is an important usability heuristic pro-

posed by Nielsen (1994). The system should speak the user’s

language, with words and concepts that are familiar to the user,

and should follow real-world conventions, presenting informa-

tion in a natural and logical order. Most children have indeed

preferred the closet metaphor. We believe that the current

nested variation of the closet metaphor is more interesting and

fun than the previous cabinet variation (Katz & Vaserman,

2009), and is a better imitation of a playroom closet that chil-

dren are familiar with. On the other hand, the tree metaphor

seemed less familiar and was also more monotonous; using the

same objects at each hierarchical level (nodes on paths that are

made of branches). We refer to more heterogeneous design

ideas for the tree metaphor later.

In summary, the results of the current study show superiority

of the closet metaphor in user preference and in several objec-

tive parameters of performance efficiency, but not in the per-

ceived usability or in performance effectiveness. Both meta-

phoric designs were effective in achieving the users’ goals, and

were similarly perceived as easy to use, but the closet metaphor

stimulated a better user experience, leading the children to pre-

fer it over the tree metaphor. User experience (UX) emphasizes

the importance of understanding how the user feels about the

system, in opposed to considering only the mere usability, with

its emphasis on user performance. Although the main focus was

to test the suitability of the metaphors in terms of usability, the

results are in line with the shift from a cognitive and functional

focus on computer applications towards an experiential affec-

tive view (Tractinsky et al., 2000; Dillon, 2001; De Angeli et

al., 2002; Hassenzahl, 2003), and show that UX is extremely

important . There are two distinct design objectives in HCI: one

is optimizing user performance and the other is optimizing user

satisfaction in achieving both pragmatic and hedonic goals

(Bevan, 2009). We claim that UX is most essential in systems

designed to foster a positive attitude in the learning process of

young children. De Angeli et al. (2006) found that participants

preferred an interface they evaluated as more engaging and fun,

despite an acknowledged inferior usability. We agree that while

it is indeed important to optimize user performance and to ful-

fill the pragmatic goals of the user, it is most important to opti-

mize user hedonic goals as well.

Referring to the results that show superiority of the closet

metaphor in efficiency but not in effectiveness, and also the

result of superiority in user preference for the closet, we argue

that while it is indeed important that the user accomplishes the

end goal using a system, namely performance effectiveness (i.e.,

the quality of the task solution), the system should also address

the process, namely performance efficiency (i.e., the resources

consumed in the process of achieving the goal). The effi-

ciency-effectiveness distinction is well known and respectively

matches the process-outcome levels of user experience with

technology (see for example Dillon, 2001). Yet, many studies

measure only objective aspects of the process, overlooking the

affective and emotional aspects and more attention should be

given to user choice and preference. Dillon suggests the POA

approach for viewing user experience, which introduces three

levels of user interaction: Process (action), Outcome (result),

and Affect (emotion), which deal respectively with what the

user does, what he attains and how he feels (Dillon, 2001). It is

reasonable to argue that users’ evaluations of the system are

influenced by the process of pursuing their goal, in terms of

A. KATZ

their enjoyment of the task and not only in terms of the time

consumed or the level of task accomplished.

As described, this study is a follow up to a previous study that

compared the tree and the nested metaphors (Katz & Vaser man,

2009), but is not an exact replication of the comparison, espe-

cially because the sample size, age range and also the meta-

phoric designs have changed. Therefore we can not compare

the results of the two researches and claim that one metaphor is

more appropriate than the other for a given age range. A follow

up study will be conducted to compare the current metaphoric

designs of the natural tree versus the closet with children in the

age range of the previous study (ages 7 - 11).

The study was limited to the comparison between two spe-

cific metaphoric designs. Although the closet metaphoric de-

sign was found to be superior in several important parameters,

one should not conclude from the results that the tree metaphor

is inappropriate. We stress that it is important not only to choose

a good metaphor, but also to properly design its characteristics.

The tree metaphor may be more successful if designed differ-

ently, and more creatively, perhaps with special interactive

effects that are playful or humoristic (for example see De An-

geli et al., 2006) or in the form of an online game that makes

the learning experience more fun (see Park & Park, 2010). Also,

a tree metaphor that contains different objects at different hier-

archy levels (such as branches, leafs, flowers and fruits) or

alternatively having a gradual change of branch thickness (from

the thickest to thinnest branches, imitating a real-world tree

growth) may be more interesting and fun variations of the cur-

rent. In addition, the tree metaphor may be more suitable than

the closet for other tasks or for children at ages that we did not

test.

Conclusion

We conclude that applying metaphors that are familiar to

children from their real life environments is a successful user

interface paradigm. We recommend that when designing an in-

terface that can exploit alternative metaphoric ideas, one should

test and compare them, by using the user centered design (UCD)

approach to find out which design is better, which characteris-

tics and features are more easy and intuitive to use, which are

difficult, and how children react to each metaphor as whole. In

addition, creating artifacts such as user interfaces is a very crea-

tive process, and since children are known as very creative,

collaborating with them can inspire and empower adults with

new insights to generate new ideas they would otherwise never

have thought of. Working closely with children that represent

the target audience as design partners in the application devel-

opment process from the earliest stage of the design is a good

strategy to create interfaces that are particularly suitable for

them. Involving children in the design process, conducting user

testing techniques that combine objective usability measures

with subjective measures (such as user satisfaction, attitudes,

feelings and preferences) and collecting qualitative data from

observations are highly recommended as ways for optimizing

both pragmatic and hedonic goals of the user.

Acknowledgements

We than k Ofir Marco and R avid Kahalani for their contribu-

tions in designing the metaphors, planning of the experimental

tasks, and running the pilot and the laboratory experiment.

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