Mosaic plots"

  collapse = TRUE,
  warning = FALSE,
  fig.height = 6,
  fig.width = 7,
  fig.path = "fig/tut04-",
  dev = "png",
  comment = "##"

# save some typing
knitr::set_alias(w = "fig.width",
                 h = "fig.height",
                 cap = "fig.cap")

# preload datasets ???
data(Arthritis, package="vcd")
art <- xtabs(~Treatment + Improved, data = Arthritis)
if(!file.exists("fig")) dir.create("fig")

Mosaic plots provide an ideal method both for visualizing contingency tables and for visualizing the fit--- or more importantly--- lack of fit of a loglinear model. For a two-way table, mosaic() fits a model of independence, $[A][B]$ or ~A+B as an R formula. For $n$-way tables, mosaic() can fit any loglinear model, and can also be used to plot a model fit with loglm(). See @vcd:Friendly:1994,vcd:Friendly:1999 for the statistical ideas behind these uses of mosaic displays in connection with loglinear models.

The essential idea is to recursively sub-divide a unit square into rectangular "tiles" for the cells of the table, such that the are area of each tile is proportional to the cell frequency. For a given loglinear model, the tiles can then be shaded in various ways to reflect the residuals (lack of fit) for a given model. The pattern of residuals can then be used to suggest a better model or understand where a given model fits or does not fit.

mosaic() provides a wide range of options for the directions of splitting, the specification of shading, labeling, spacing, legend and many other details. It is actually implemented as a special case of a more general class of displays for $n$-way tables called strucplot, including sieve diagrams, association plots, double-decker plots as well as mosaic plots. For details, see help(strucplot) and the "See also" links, and also @vcd:Meyer+Zeileis+Hornik:2006b, which is available as an R vignette via vignette("strucplot", package="vcd").

Example: A mosaic plot for the Arthritis treatment data fits the model of independence, ~ Treatment + Improved and displays the association in the pattern of residual shading. The plot below is produced with the following call to mosaic().

#| Arthritis1,
#| fig.height = 6,
#| fig.width = 7,
#| fig.cap = "Mosaic plot for the `Arthritis` data."
data(Arthritis, package="vcd")
art <- xtabs(~Treatment + Improved, data = Arthritis)
mosaic(art, gp = shading_max, 
            split_vertical = TRUE, 
            main="Arthritis: [Treatment] [Improved]")

gp = shading_max specifies that color in the plot signals a significant residual at a 90% or 99% significance level, with the more intense shade for 99%. Note that the residuals for the independence model were not large (as shown in the legend), yet the association between Treatment and Improved is highly significant.


In contrast, one of the other shading schemes, from @vcd:Friendly:1994 (use: gp = shading_Friendly), uses fixed cutoffs of $\pm 2, \pm 4$, to shade cells which are individually significant at approximately $\alpha = 0.05$ and $\alpha = 0.001$ levels, respectively. The right panel below uses gp = shading_Friendly.

mosaic(art, gp = shading_Friendly, 
            split_vertical = TRUE, 
            main="Arthritis: gp = shading_Friendly")

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vcdExtra documentation built on April 21, 2022, 5:10 p.m.