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 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 Meyer, Zeileis, & Hornik (2006), which is available as an R vignette via
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
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
Improved is highly significant.
summary(art) ## Call: xtabs(formula = ~Treatment + Improved, data = Arthritis) ## Number of cases in table: 84 ## Number of factors: 2 ## Test for independence of all factors: ## Chisq = 13.055, df = 2, p-value = 0.001463
In contrast, one of the other shading schemes, from 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")