SPSS Categories® 11.0

6 Chapter 1
Although many coding schemes for a variable are functionally equivalent, schemes
with small differences between codes are preferred because the codes have an impact on
the amount of output produced by a procedure. All categories coded with values
between 1 and the user-defined maximum are valid. If any of these categories are empty,
the corresponding quantifications will be either system missing or zero, depending on
the procedure. Although neither of these assignments affect the analyses, output is produced
for these categories. Thus, for scheme B, job has four categories that receive
system-missing values. For scheme C, there are also four categories receiving systemmissing
indicators. In contrast, for scheme A there are no system-missing quantifications.
Using consecutive integers as codes for variables treated as nominal or ordinal
results in much less output without affecting the results.
Coding schemes for variables treated as numerical are more restricted than the ordinal
case. For these variables, the differences between consecutive categories are
important. Table 1.3 displays three coding schemes for age.
Table 1.3 Alternative coding schemes for age
Category
Scheme
A B C
20 20 1 1
22 22 3 2
25 25 6 3
27 27 8 4
Any recoding of numerical variables must preserve the differences between the categories.
Using the original values is one method for ensuring preservation of differences.
However, this can result in many categories having system-missing indicators. For example,
scheme A in Table 1.3 employs the original observed values. For all Categories
procedures except for correspondence analysis, the maximum category value is 27 and
the minimum category value is set to 1. The first 19 categories are empty and receive
system-missing indicators. The output can quickly become rather cumbersome if the
maximum category is much greater than 1 and there are many empty categories between
1 and the maximum.
To reduce the amount of output, recoding can be done. However, in the numerical
case, the Automatic Recode facility should not be used. Coding to consecutive integers
results in differences of 1 between all consecutive categories and as a result, all quantifications
will be equally spaced. The metric characteristics deemed important when
treating a variable as numerical are destroyed by recoding to consecutive integers. For
example, scheme C in Table 1.3 corresponds to automatically recoding age. The difference
between categories 22 and 25 has changed from three to one, and the
quantifications will reflect the latter difference.
An alternative recoding scheme that preserves the differences between categories is to
subtract the smallest category value from every category and add one to each difference.