<s>
In	O
human	O
–	O
computer	O
interaction	O
,	O
the	O
keystroke-level	B-General_Concept
model	I-General_Concept
(	O
KLM	B-General_Concept
)	O
predicts	O
how	O
long	O
it	O
will	O
take	O
an	O
expert	O
user	O
to	O
accomplish	O
a	O
routine	O
task	O
without	O
errors	O
using	O
an	O
interactive	O
computer	O
system	O
.	O
</s>
<s>
The	O
keystroke-level	B-General_Concept
model	I-General_Concept
is	O
still	O
relevant	O
today	O
,	O
which	O
is	O
shown	O
by	O
the	O
recent	O
research	O
about	O
mobile	O
phones	O
and	O
touchscreens	O
(	O
see	O
Adaptions	O
)	O
.	O
</s>
<s>
The	O
keystroke-level	B-General_Concept
model	I-General_Concept
consists	O
of	O
six	O
operators	O
:	O
the	O
first	O
four	O
are	O
physical	O
motor	O
operators	O
followed	O
by	O
one	O
mental	O
operator	O
and	O
one	O
system	O
response	O
operator	O
:	O
</s>
<s>
The	O
KLM	B-General_Concept
is	O
based	O
on	O
the	O
keystroke	O
level	O
,	O
which	O
belongs	O
to	O
the	O
family	O
of	O
GOMS	B-General_Concept
models	O
.	O
</s>
<s>
The	O
KLM	B-General_Concept
and	O
the	O
GOMS	B-General_Concept
models	O
have	O
in	O
common	O
that	O
they	O
only	O
predict	O
behaviour	O
of	O
experts	O
without	O
errors	O
,	O
but	O
in	O
contrast	O
the	O
KLM	B-General_Concept
needs	O
a	O
specified	O
method	O
to	O
predict	O
the	O
time	O
because	O
it	O
does	O
not	O
predict	O
the	O
method	O
like	O
GOMS	B-General_Concept
.	O
</s>
<s>
Therefore	O
,	O
the	O
KLM	B-General_Concept
has	O
no	O
goals	O
and	O
method	O
selection	O
rules	O
,	O
which	O
in	O
turn	O
makes	O
it	O
easier	O
to	O
use	O
.	O
</s>
<s>
The	O
KLM	B-General_Concept
resembles	O
the	O
model	O
K1	O
from	O
the	O
family	O
of	O
GOMS	B-General_Concept
models	O
the	O
most	O
because	O
both	O
are	O
at	O
the	O
keystroke	O
level	O
and	O
possess	O
a	O
generic	O
M	O
operator	O
.	O
</s>
<s>
The	O
difference	O
is	O
that	O
the	O
M	O
operator	O
of	O
the	O
KLM	B-General_Concept
is	O
more	O
aggregated	O
and	O
thus	O
larger	O
(	O
1.35	O
seconds	O
vs.	O
0.62	O
seconds	O
)	O
,	O
which	O
makes	O
its	O
mental	O
operator	O
more	O
similar	O
to	O
the	O
CHOOSE	O
operations	O
of	O
the	O
model	O
K2	O
.	O
</s>
<s>
All	O
in	O
all	O
,	O
the	O
KLM	B-General_Concept
represents	O
the	O
practical	O
use	O
of	O
the	O
GOMS	B-General_Concept
keystroke	O
level	O
.	O
</s>
<s>
The	O
KLM	B-General_Concept
was	O
designed	O
to	O
be	O
a	O
quick	O
and	O
easy	O
to	O
use	O
system	O
design	O
tool	O
,	O
which	O
means	O
that	O
no	O
deep	O
knowledge	O
about	O
psychology	O
is	O
required	O
for	O
its	O
usage	O
.	O
</s>
<s>
Also	O
,	O
task	O
times	O
can	O
be	O
predicted	O
(	O
given	O
the	O
limitations	O
)	O
without	O
having	O
to	O
build	O
a	O
prototype	B-Application
,	O
recruit	O
and	O
test	O
users	O
,	O
which	O
saves	O
time	O
and	O
money	O
.	O
</s>
<s>
See	O
the	O
example	O
for	O
a	O
practical	O
use	O
of	O
the	O
KLM	B-General_Concept
as	O
a	O
system	O
design	O
tool	O
.	O
</s>
<s>
The	O
keystroke-level	B-General_Concept
model	I-General_Concept
has	O
several	O
restrictions	O
:	O
</s>
<s>
If	O
this	O
is	O
crucial	O
,	O
a	O
GOMS	B-General_Concept
model	O
has	O
to	O
be	O
used	O
(	O
e.g.	O
</s>
<s>
This	O
implies	O
that	O
the	O
KLM	B-General_Concept
is	O
best	O
suited	O
for	O
short	O
tasks	O
with	O
few	O
operators	O
.	O
</s>
<s>
In	O
addition	O
,	O
the	O
KLM	B-General_Concept
can	O
not	O
make	O
a	O
perfect	O
prediction	O
and	O
has	O
a	O
root-mean-square	O
error	O
of	O
21%	O
.	O
</s>
<s>
The	O
following	O
example	O
slightly	O
modified	O
to	O
be	O
more	O
compact	O
from	O
Kieras	O
shows	O
the	O
practical	O
use	O
of	O
the	O
KLM	B-General_Concept
by	O
comparing	O
two	O
different	O
ways	O
to	O
delete	O
a	O
file	O
for	O
an	O
average	O
skilled	O
typist	O
.	O
</s>
<s>
Note	O
that	O
M	O
is	O
1.35	O
seconds	O
as	O
stated	O
in	O
the	O
KLM	B-General_Concept
instead	O
of	O
1.2	O
seconds	O
used	O
by	O
Kieras	O
.	O
</s>
<s>
The	O
six	O
operators	O
of	O
the	O
KLM	B-General_Concept
can	O
be	O
reduced	O
,	O
but	O
this	O
decreases	O
the	O
accuracy	O
of	O
the	O
model	O
.	O
</s>
<s>
While	O
the	O
existing	O
KLM	B-General_Concept
applies	O
to	O
desktop	O
applications	O
,	O
the	O
model	O
might	O
not	O
fulfill	O
the	O
range	O
of	O
mobile	O
tasks	O
,	O
or	O
as	O
Dunlop	O
and	O
Cross	O
declaimed	O
KLM	B-General_Concept
is	O
no	O
longer	O
precise	O
for	O
mobile	O
devices	O
.	O
</s>
<s>
There	O
are	O
various	O
efforts	O
to	O
extend	O
the	O
KLM	B-General_Concept
regarding	O
the	O
use	O
for	O
mobile	O
phones	O
or	O
touch	O
devices	O
.	O
</s>
<s>
While	O
Li	O
and	O
Holleis	O
both	O
agree	O
that	O
the	O
KLM	B-General_Concept
model	O
can	O
be	O
applied	O
to	O
predict	O
task	O
times	O
on	O
mobile	O
devices	O
,	O
Li	O
suggests	O
further	O
modifications	O
to	O
the	O
model	O
,	O
by	O
introducing	O
a	O
new	O
concept	O
called	O
operator	O
blocks	O
.	O
</s>
<s>
These	O
are	O
defined	O
as	O
"	O
the	O
sequence	O
of	O
operators	O
that	O
can	O
be	O
used	O
with	O
high	O
repeatability	O
by	O
analyst	O
of	O
the	O
extended	O
KLM.	O
”	O
.	O
</s>
