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2 RELATED WORK
Human Mental Workload (HMW) is a multifaceted
complex construct mainly applied in psychology and
other cognitive sciences. A plethora of definitions ex-
ists in the literature (Hancock and Meshkati, 1988;
Wickens, 1987; Cain, 2007; Gopher and Donchin,
1986). Intuitively, mental workload or cognitive
workload is the amount of mental work necessary for
a person to complete a task over a given period of
time. Generally, it is not an inherent property, rather
it emerges from the interaction between the require-
ments of a task, the circumstances under which it is
performed, and the skills, behaviors and perceptions
of the operator. The operational and practical na-
ture of the construct of human mental workload, in
the last few decades, has been acquiring interest in
Neuro-science, Physiology and even Computer sci-
ence (Kramer and Sirevaag, 1987; Kantowitz, 1988;
Donnell and Eggemeier, 1998; Young and Stanton,
2001). There is a wide application field (Donnell and
Eggemeier, 1998; Tracy and Albers, 2006; Xie and
Salvendy, 2000; Gwizdka, 2009a; Gwizdka, 2009b)
and this new research domain may have an important
impact in the future, above all in Human-Computer
Interaction. The concept has become increasingly
important since modern interactive systems and in-
terfaces may impose severe requirements on mental
workload or information-processing capabilities.
There exist three major types of mental work-
load measures: performance-based, subjective and
physiological. The rationale behind performance-
based measures is that performance on a selected sec-
ondary task will decrease as a function of the de-
mands of a selected primary task. Subjective mea-
sures include self-assessments using uni-dimensional
or multi-dimensional scales. The former consider a
measures of overall mental workload, the latter take
into consideration individual dimensions of mental
workload, therefore being more accurate in determin-
ing the source of any potential workload problem.
Physiological measures are based on the premise that
mental workload will generate changes in the body
such as pupil dilation, changes in skin conductance,
body pressure and heart rate. Although they are ac-
curate and can work on a continuous scale, the equip-
ment they require is generally impractical for experi-
ments as it requires trained staff.
In this paper we focus on subjective multi-
dimensional measures and we use three well-
established tools: The NASA Task Load Index
(NASA-TLX) (Kantowitz, 1988); The Simplified
Subjective Workload Assessment Technique (SWAT)
(Luximon and Goonetilleke, 2001); The Workload
Profile (WP) (Tsang and Velazquez, 1996). In the
following paragraphs, we briefly describe each tech-
nique, introducing the formal models in section 3.
NASA-TLX (Hart, 2006) uses six dimensions to
estimate mental workload: mental demand, physical
demand, temporal demand, performance, effort and
frustration. Each of these is in a scale from 0 to 100.
The final mental workload index is a weighted av-
erage of the six areas that provide an overall score.
The weights are obtained via a paired comparisons
which requires the operator to choose which dimen-
sion, across all pairs with the six dimensions, is more
relevant to mental workload. The number of times a
dimension has been chosen by the operator represents
the weight of that dimension scale, for a given task
(Kantowitz, 1988).
SWAT is a subjective multi-dimensional rating
procedure that uses three areas to evaluate mental
workload: time load, mental effort load, psychologi-
cal stress load, each of them in a three-levels scale. In
this paper we have adopted a simplified version of the
SWAT model, the Continuous SWAT dimensions with
weight (Luximon and Goonetilleke, 2001) which uses
a paired comparisons among the three dimensions ex-
actly as in the NASA-TLX model. The final mental
workload is the average of the weighted areas (Luxi-
mon and Goonetilleke, 2001).
WP is a subjective workload assessment tech-
nique, based on the Multiple Resource Theory (MRT)
of Wickens (Wickens, 1987). In this procedure eight
dimensions are considered: perceptual/central pro-
cessing, response selection and execution, spatial pro-
cessing, verbal processing, visual processing, audi-
tory processing, manual output and speech output.
The WP procedure asks the operators to provide the
proportion of attentional resources, in the range 0 to 1,
used after the execution of a task. The overall work-
load rating is computed summing each of the 8 scores.
The three subjective techniques have low imple-
mentation requirements along with low intrusiveness
and high subject acceptability. These peculiarities
have promoted new research in which the construct of
Human Mental Workload has been adopted for evalu-
ating alternative interfaces. Tracy and Albers adopted
three different techniques for measuring mental work-
load applied to web-site design: NASA-TLX, The
Sternberg Memory Test and a tapping test (Tracy and
Albers, 2006) (Albers, 2011). They proposes a tech-
nique to individuate sub-areas of a web-site, in which
end-users manifested higher mental workload during
interaction. In turn, this allowed designers to mod-
ify those critical regions for enhancing their interface.
Zhu and Hou (Zhu and Hou, 2009) noted how roles
can be useful in interface design and proposed a role-
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