This menu sets a number of SPAN options. The options are retained and written as
a logical sequence in the registration file register.txt.
17.1 Full Boolean reduction
Boolean expressions in SPAN are automatically simplified according to attributes
in a Boolean expression. The simplification does not take account of the nature
of variables from which attributes are constructed unless this item is checked.
For example the expression X1ÇX1ÇX1ÇX2
reduces to X1ÇX2
regardless of the nature of the way the attributes are constructed. If, however,
it is known that X1
and X2 are
attributes derived from some underlying nominal variable, x, such that
X1 = {x
= 1} and X2
= {x = 2} then X1ÇX1ÇX1ÇX2 is the null
set.
Full Boolean reductions are those simplifications to Boolean expressions that
arise because of the way attributes are constructed, as in the above example. These
reductions may arise if several attributes are created from a variable either by
specifying a series of cuts or by repeat lines in the control file. This option
will simplify all such Boolean expressions. More details are in
Appendix A5.4.
17.2 Attributes are primitive
If this option is in place no simplification of Boolean expressions is
done.
The option would not normally be used, but it is useful, for example, to investigate
the relationships between attributes using subgroup rectangle diagrams. For example
if you wanted to display rectangles at different cutoffs of systolic blood
pressure:
A=(sbp>140) or (sbp>160) or (sbp>180)
which would otherwise simplify to (sbp>40).
17.3 Tied partitions: use last found
Partitions are tied if they have the same complexity and effectiveness.
With this option the last found tied partition during a search supplants the existing
one.
17.4 Missing attributes
Missing attributes arise from missing values of variables used to create the attributes
(see 8.5). They can be handled in three ways when evaluating a partition:
- Skip if any missing in attribute set
If any attribute is missing in the defined attribute set, the entire observation
is skipped (ignored). With this option the sample size for a partition is the same
as for any other partition derived from the attribute set.
- Skip if any missing in combination
If any attribute is missing in a defined Boolean combination, the observation is
skipped. With this option sample sizes for one partition may differ from that of
another, depending on the pattern of missing observations.
- Skip if combination not evaluable
Sometimes it is possible to evaluate a Boolean expression even if some of its attributes
are missing. For example, suppose you need to determine whether an observation lies
in A = X1ÈX2.
If X2 is not
missing then knowledge of the value of X1 is immaterial when X2 is present.
Whether a Boolean expression can be evaluated when some of its constituent attributes
are missing, will thus depend on the value of the attributes which are not missing.
Another example is A = (X1ÇX2)È(X3ÇX4). A would be present
if X1ÇX2 occurred, regardless
of whether X3
and X4 were
missing, but its value would be indeterminate if X1 was present and X2 absent with X3 and X4
missing.
The third strategy has the advantage that it may utilise more information, but should
be used warily if missing values do not occur at random and are closely associated
with the dependent variable(s).
When a partition is accessed from the Process menu, a message is sent informing
of the number of missing observations due to missing attributes.
17.5 Detailed output
This option sends fuller details of a SPAN session to the output log. Usually you
would not want this option, since the output log file can become very large.
17.6 Display graphics while searching
If this option is checked points on the complexity hull are added as they are generated
by the search. The option may slow down the search and is best not used.
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