IBM Switch 15 Manual de usuario descargar pdf (Pagina 109)

Chapter

Handling Missing Values

Overview of Missing Values

During the Data Preparation phase of data mining, you will of ten want to replace missing values

in th e data. Missing values are values in the data set that are unknown, uncolle cted, or incorrectly

entered. Usually, such values are invalid for th eir ﬁelds. For example, the ﬁeld Sex should contain

the values M and F. If y ou discover the values Y or Z in the ﬁeld, you can safely assume that such

values are invalid and should therefore be interpret ed as blanks. Likew ise, a negative value for the

ﬁeld Age is meaningless and should also be interpreted as a blank . Frequently, such obviously

wrong values are purpose ly ente r ed, or ﬁelds left blank, during a ques tionnaire to indicate a

nonresp onse. At times, you may want to examine these blanks more closely to determine whether

a nonresponse, s uch as the re f usal to give one’s age, is a factor in pr edicting a speciﬁc o utcome.

Some modeling techniques handle missin g data better than others. For example, C5.0 and

Apriori cope w ell with values that are explicitly declared as “missing” in a Type node. Other

modeling techniques have trouble dealing with missing values and experience longer training

times, res ulting in less-accurate models.

There are several ty pe s of missing values recognized by IBM® SPSS® Modeler:

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Null or system-missing values.

These are nonstring values that have been left blank in the

database or source ﬁle a nd have not been speciﬁcally deﬁned as “missing” in a source or

Type nod e. System-missing values are displayed as $null$. Note that empty strings are not

considered nu lls in SPSS Modeler, although they may be trea ted as nulls by certain databases.

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Empty strings and white space.

Empty string values and white space (str ings with no visible

characters) are treated as distinct from null v alues. Empty strings are treated as equivalent to

white space for mo st purposes. For example, if you select the option to treat white space as

blanks in a source or Type node, this s etting applies to emp ty strings as well.

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Blank or user-defined missing values.

These are values such as unknown, 99, or –1 that ar e

explicitly de ﬁned in a source node or Type node as missing. Optionally, you can also choose

to tre at nulls a nd white space as blanks, which allows them to be ﬂagged for spec ial treatment

and to be excluded from most calcula tions. Fo r example, you can use the @BLANK function to

treat these values, along with other types of miss ing values, as blanks.

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IBM Switch 15 Manual de usuario Pagina 109