Database Reference
In-Depth Information
3.1.
Relative Predicates
A natural way of describing series is to indicate when they increase, decrease
or stay. These predicates deal with these concepts:
•
increases(Example, Variable, Beginning, End, Value)
. It is true, for the
Example
, if the difference between the values of the
Variable
for
End
and
Beginning
is greater or equal than
Value
.
•
decreases(Example, Variable, Beginning, End, Value)
.
•
stays(Example, Variable, Beginning, End, Value)
. It is true, for the
Example
,
if the range of values of the
Variable
in the interval is less or equal than
Value
.
Frequently, series are noisy and, hence, a strict definition of
increases
and
decreases
in an interval i.e., the relation holds for all the points in the
interval, is not useful. It is possible to filter the series prior to the learning
process, but we believe that a system for time series classification must not
rely on the assumption that the data is clean. For these two predicates we
consider what happens only in the extremes of the interval. The parameter
Value
is necessary for indicating the amount of change.
For the predicate
stays
it is neither useful to use a strict definition. In
this case all the points in the interval are considered. The parameter
Value
is used to indicate the maximum allowed difference between the values in
the interval.
3.2.
Region Based Predicates
The selection and definition of these predicates is based in the ones used in
a visual rule language for dynamic systems [Alonso Gonzalez and Rodr ıguez
Diez (1999)]. These predicates are:
•
always(Example, Variable, Region, Beginning, End)
. It is true, for the
Exam-
ple
,ifthe
Variable
is always in this
Region
in the interval between
Begin-
ning
and
End
.
•
sometimes(Example, Variable, Region, Beginning, End)
.
•
true percentage(Example, Variable, Region, Beginning, End, Percentage)
.It
is true, for the
Example
, if the percentage of the time between
Beginning
and
End
where the variable is in
Region
is greater or equal to
Percentage
.
Once that it is decided to work with temporal intervals, the use and
definition of the predicates
always
and
sometimes
is natural, due to the fact
that they are the extension of the conjunction and disjunction to intervals.
