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SPSS Categories® 11.0

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4 Chapter 1<br />

Transformation Plots<br />

The different levels at which each variable can be scaled impose different restrictions on<br />

the quantifications. Transformation plots illustrate the relationship between the quantifications<br />

and the original categories resulting from the selected optimal scaling level.<br />

For example, a linear transformation plot results when a variable is treated as numerical.<br />

Variables treated as ordinal result in a nondecreasing transformation plot. Transformation<br />

plots for variables treated nominally that are U-shaped (or the reverse) display a<br />

quadratic relationship. Nominal variables could also yield transformation plots without<br />

apparent trends by changing the order of the categories completely. Figure 1.1 displays<br />

a sample transformation plot.<br />

Transformation plots are particularly suited to determining how well the selected<br />

optimal scaling level performs. If several categories receive similar quantifications, collapsing<br />

these categories into one category may be warranted. Alternatively, if a variable<br />

treated as nominal receives quantifications that display an increasing trend, an ordinal<br />

transformation may result in a similar fit. If that trend is linear, numerical treatment may<br />

be appropriate. However, if collapsing categories or changing scaling levels is warranted,<br />

the analysis will not change significantly.<br />

Figure 1.1 Transformation plot of price (numerical)<br />

Although HOMALS does not currently offer the transformation plot in Figure 1.1 as an<br />

option, creating this plot is a straightforward procedure. For each variable, create a new<br />

variable containing the values (and labels) of the categories. Create a new variable containing<br />

the quantifications for that variable from the Categories analysis. Use the line facility<br />

with the data in the chart representing the values of individual cases. You can use<br />

the quantifications for the line in the chart and use the categories to label the chart.

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