This vignette shows how to model multivariate distributions with
copulas
using univariateML
and the copula
package.
A copula is a function describing the dependency among a set of one-dimensional distributions. If both the marginal distributions and the copula is known, the entire multivariate distribution is known too.
Suppose we look at a multivariate distribution as the pair of a copula and its marginals. Then a natural model selection method is to
This two-step procedure is commonly used due to its simplicity. The procedure must be carried out this order since the marginal data cannot be transformed to the unit interval unless we know the marginal distributions.
The univariateML
can be used for task 1, while the copula
package can be used to do task 2.
The abalone
data set is included in this package. It consists of $9$ physical
measurements of $4177$ sea snails.
library("univariateML") head(abalone)
Following @ko2019focused we will take a look at four measurements of the abalones,
namely diameter
, height
, shell_weight
and age
. The variable age
is not
present in the abalone
data, but is defined as age = rings + 1.5
. Moreover,
there are two outliers in the height
data at at $1.13$ and $0.52$. We will
remove these outliers and all columns we don't need in the following.
data = dplyr::filter(abalone, height < 0.5) data$age = data$rings + 1.5 data = data[c("diameter", "height", "shell_weight", "age")] hist(data$height, main = "Abalone height", xlab = "Height in mm")
Let's continue doing step 1. First we must decide on a set of models to try out.
models = c("gumbel", "laplace", "logis", "norm", "exp", "gamma", "invgamma", "invgauss", "invweibull", "llogis", "lnorm", "rayleigh", "weibull", "lgamma", "pareto", "beta", "kumar", "logitnorm") length(models)
Optionally, we can use all implemented models with
univariateML_models
The next step is to fit all models, compute the AIC, and select the best model.
This is exactly what model_select()
does.
margin_fits <- lapply(data, model_select, models = models, criterion = "aic")
Now we use the fitCopula
from the package copula
on the transformed margins of abalone
.
We will examine two elliptical copulas and three Archimedean copulas. The elliptical copulas are the Gaussian copula and the t-copula, while the Archimedean copulas are the Joe copula, the Clayton copula, and the Gumbel copula.
# Transform the marginals to the unit interval. y = sapply(seq_along(data), function(j) pml(data[[j]], margin_fits[[j]])) # The copulas described above. copulas = list(normal = copula::normalCopula(dim = 4, dispstr = "un"), t = copula::tCopula(dim = 4, dispstr = "un"), joe = copula::joeCopula(dim = 4), clayton = copula::claytonCopula(dim = 4), gumbel = copula::gumbelCopula(dim = 4)) fits = sapply(copulas, function(x) copula::fitCopula(x, data = y, method = "mpl")) sapply(fits, AIC)
The t-copula is the clear winner of the AIC competition. The Archimedean copulas perform particularly poorly.
Hence our final model is the t-copula with Kumaraswamy (mlkumar
) marginal distribution for
diameter
, normal marginal distribution for height
, Weibull marginal distribution for shell_weight
,
and log normal marginal distribution for age
.
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