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1. Exploratory Data Analysis
1.3. EDA Techniques
1.3.3. Graphical Techniques: Alphabetic Scatter Plot

Conditioning Plot

Check pairwise relationship between two variables conditional on a third variable
A conditioning plot, also known as a coplot or subset plot, is a plot of two variables conditional on the value of a third variable (called the conditioning variable). The conditioning variable may be either a variable that takes on only a few discrete values or a continuous variable that is divided into a limited number of subsets.

One limitation of the scatterplot matrix is that it cannot show interaction effects with another variable. This is the strength of the conditioning plot. It is also useful for displaying scatter plots for groups in the data. Although these groups can also be plotted on a single plot with different plot symbols, it can often be visually easier to distinguish the groups using the conditioning plot.

Although the basic concept of the conditioning plot matrix is simple, there are numerous alternatives in the details of the plots.

  1. It can be helpful to overlay some type of fitted curve on the scatter plot. Although a linear or quadratic fit can be used, the most common alternative is to overlay a lowess curve.

  2. Due to the potentially large number of plots, it can be somewhat tricky to provide the axis labels in a way that is both informative and visually pleasing. One alternative that seems to work well is to provide axis labels on alternating rows and columns. That is, row one will have tic marks and axis labels on the left vertical axis for the first plot only while row two will have the tic marks and axis labels for the right vertical axis for the last plot in the row only. This alternating pattern continues for the remaining rows. A similar pattern is used for the columns and the horizontal axis labels. Note that this approach only works if the axes limits are fixed to common values for all of the plots.

  3. Some analysts prefer to connect the scatter plots. Others prefer to leave a little gap between each plot. Alternatively, each plot can have its own labeling with the plots not connected.

  4. Although this plot type is most commonly used for scatter plots, the basic concept is both simple and powerful and extends easily to other plot formats.
Sample Plot conditioning plot of torque versus time for six values of temperature

In this plot of of the PR1.DAT data set, temperature has six distinct values. We plot torque versus time for each of these temperatures. This example is discussed in more detail in the process modeling chapter.

Definition Given the variables X, Y, and Z, the conditioning plot is formed by dividing the values of Z into k groups. There are several ways that these groups may be formed. There may be a natural grouping of the data, the data may be divided into several equal sized groups, the grouping may be determined by clusters in the data, and so on. The page will be divided into n rows and c columns where nck. Each row and column defines a single scatter plot.

The individual plot for row i and column j is defined as

  • Vertical axis: Variable Y
  • Horizontal axis: Variable X
where only the points in the group corresponding to the ith row and jth column are used.
Questions The conditioning plot can provide answers to the following questions:
  1. Is there a relationship between two variables?
  2. If there is a relationship, does the nature of the relationship depend on the value of a third variable?
  3. Are groups in the data similar?
  4. Are there outliers in the data?
Related Techniques Scatter plot
Scatterplot matrix
Locally weighted least squares
Software Conditioning plots are becoming increasingly common in general purpose statistical software programs, including R and Dataplot. If a software program does not generate conditioning plots, but it does provide multiple plots per page and scatter plots, it should be possible to write a macro to generate a conditioning plot.
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