An R package for Non-Normal Multivariate Distributions: Simulation and Probability Calculations from Multivariate Lomax (Pareto Type II) and Other Related Distributions

Convenient and easy-to-use programs are readily available in R to simulate data from and probability calculations for several common multivariate distributions such as normal and \(t\). However, functions for doing so from other less common multivariate distributions, especially those which are asymmetric, are not as readily available, either in R or otherwise. We introduce the R package NonNorMvtDist to generate random numbers from multivariate Lomax distribution, which constitutes a very flexible family of skewed multivariate distributions. Further, by applying certain useful properties of multivariate Lomax distribution, multivariate cases of generalized Lomax, Mardia’s Pareto of Type I, Logistic, Burr, Cook-Johnson’s uniform, \(F\), and inverted beta can be also considered, and random numbers from these distributions can be generated. Methods for the probability and the equicoordinate quantile calculations for all these distributions are then provided. This work substantially enriches the existing R toolbox for nonnormal or nonsymmetric multivariate probability distributions.

1 Introduction

A \(k\)-dimensional multivariate Lomax (Pareto Type II) probability distribution was first introduced by (Nayak 1987) as a joint distribution of \(k\) skewed nonnegative random variables \(X_1,\cdots, X_k\) with joint probability density function given by \[\begin{aligned} f(x_1, \dots, x_k) = \frac{\left[ \prod_{i=1}^{k} \theta_i\right] a(a+1) \cdots (a+k-1)}{\left(1+\sum_{i=1}^{k} \theta_i x_i \right)^{a+k}}, \quad x_i > 0, a, \theta_i>0, i=1,\dots,k. \label{ML} \end{aligned} \tag{1}\] We will denote above density function by ML\(_k(a; \theta_1,\dots, \theta_k)\). Prior to Nayak (1987), the bivariate case of multivariate Lomax distribution was studied by Lindley and Singpurwalla (1986). Nayak (1987) indicated that the \(k\)-dimensional multivariate Lomax distribution could be obtained by mixing \(k\) independent univariate exponential distributions with different failure rates with the mixing parameter \(\eta\) that has a gamma distribution with certain shape parameter \(a\) and the scale parameter \(1\). This fact readily provides an approach to simulate the multidimensional random vectors from the multivariate Lomax distribution. The multivariate Lomax distribution is also transformable to many other useful multivariate distributions, and therefore, simulations from these distributions are also easily accomplished. Similarly, with appropriate transformations or reparameterizations (or otherwise directly from the probability density function (\(pdf\))), we can also accomplish the cumulative probability calculations as well as the calculation of equicoordinate quantiles. The objective of this work is to formalize all of the above and to provide a ready-to-use R package titled NonNorMvtDist for practitioners to efficiently execute the same. See Lun and Khattree (2020).

The objective of our work is to enrich the existing R packages for supporting simulation and computations for nonnormal continuous multivariate distributions. To the best of our knowledge and also from the list of packages for multivariate distributions in CRAN (https://cran.r-project.org/web/views/Distributions.html), there is no package that provides both simulation and probability computations for multivariate Pareto distribution. In our package, we provide functions for doing so to both Lomax (Pareto type II) and Mardia’s Pareto type I distributions. For multivariate logistic distribution, package VGAM (Yee 2019) implements the bivariate logistic distribution while we support \(p\)-variate logistic distribution for \(p>2\). Moreover, multivariate Burr, \(F\), and inverted beta distributions had not been implemented in R until we included them in the package NonNorMvtDist.

This paper is organized as follows. In Section 2, we provide simulation algorithms for generating data from \(k\)-dimensional multivariate Lomax and generalized Lomax (to be defined later) distributions. Through transformations, the tasks of random numbers generation from Mardia’s multivariate Pareto Type I, Logistic, Burr, Cook-Johnson’s uniform, and \(F\) are achieved. Based on the remarks from Balakrishnan and Lai (2009), we then extend Nayak’s work to simulate data from the multivariate inverted beta distribution. In Section 3, we discuss numerical computations of cumulative distribution functions of equicoordinate quantiles and survival functions for the above distributions. In Section 4, we illustrate the use of respective functions as implemented in R for each of the above distributions for the bivariate (\(k=2\)) case. In Section 5, we provide a run-time study to assess the computation times for functions as the dimension of data increases. In Section 6, we implement maximum likelihood estimation of parameters for these distributions. In Section 7, we give two applications of package NonNorMvtDist, namely, (i) data generation from certain nonelliptical symmetric multivariate distributions with univariate normal marginals and (ii) computation of critical values of the multivariate \(F\) distribution. Section 8 includes some concluding remarks, pointing out other applications.

2 Multivariate Lomax and related distributions

Multivariate Lomax distribution can be derived as the probability distribution of a \(k\)-component system where \(k\) independent exponential random variables have a common environment or mixing parameter following a gamma distribution with shape parameter \(a\) and scale parameter \(b\). Let the corresponding random vector be \(\mathbf{X}=(X_1,\cdots,X_k)'\). The probability density function of \(\mathbf{X}\) is given by ((1)), and the joint survival function of \(\mathbf{X}\) is \[\begin{aligned} S(x_1, \dots, x_k) = \left( 1+\sum_{i=1}^{k} \theta_i x_i \right)^{-a}, \quad x_i > 0, a>0, \theta_i>0, i=1,\dots,k. \label{Eq:MLSv} \end{aligned} \tag{2}\] Specifically, the pivotal result that we use is given by the following theorem (see Nayak (1987)),
Theorem 2.1: Conditioned on fixed mixing parameter \(\eta\), representing the environment effect, let \(X_1, \cdots, X_k\) be independent exponentially distributed random variables with failure rates \(\eta \lambda_1, \cdots, \eta \lambda_k\), respectively. Let the environment effect \(\eta\) be distributed as a Gamma random variate with probability density \[\begin{aligned} g(\eta) = b^a \exp(-\eta b) \eta^{a-1}/\Gamma(a), \eta > 0, a,b > 0. \end{aligned}\] Then, the unconditional joint density of \(X_1, \cdots,X_k\) is given by ((1)), where \(\theta_i = \lambda_i/b, i=1,\cdots,k\). Clearly, without loss of generality, \(b\) can be taken as 1, in which case \(\theta_i=\lambda_i\), \(i=1,\cdots, k\).

In view of the above result, we implement the simulation from \(k\)-dimensional multivariate Lomax distribution by adopting the following algorithm.

Algorithm-ML\(_k(a; \theta_1,\dots, \theta_k)\):

1. Generate a random number \(\eta\) from Gamma\((a,1)\) distribution;

2. With \(\eta\) as generated in Step 1, generate \(k\)-independent random variables \(X_i\), \(i=1,\dots,k\), each from exponential distribution with parameter \(\eta \theta_i\), \(i=1,\dots,k\), respectively. Let \(\mathbf{X}=(X_1,X_2,\cdots,X_k)'\);

3. To obtain a random sample of size \(n\), repeat the Steps 1 and 2 \(n\) times.

Nayak (1987) also generalized this distribution by mixing conditionally independent \(X_i\) having the Gamma\((l_i, \eta \theta_i)\) distribution, with mixing variable \(\eta \sim \text{Gamma}(a,1)\), \(i=1,\dots,k\). This is termed as generalized multivariate Lomax distribution denoted by GML\((a; \theta_1,\dots, \theta_k, l_1,\dots,l_k)\) and has the probability density function \[\begin{aligned} \label{GML} f(x_1, \cdots, x_k) = \frac{\left[ \prod_{i=1}^{k} \theta_i^{l_i}\right] \Gamma\left(\sum_{i=1}^{k} l_i + a \right) \prod_{i=1}^{k} x_i^{l_i-1}}{\Gamma(a) \left[ \prod_{i=1}^{k} \Gamma(l_i) \right] \left(1+\sum_{i=1}^{k} \theta_i x_i \right)^{\sum_{i=1}^{k} l_i + a}}, \quad x_i>0, a,\theta_i, l_i>0, i=1,\cdots,k \end{aligned} \tag{3}\] Accordingly, we perform the corresponding simulation by implementing the suitable changes in the above algorithm. The algorithm is given below.

Algorithm-GML\((a; \theta_1,\dots, \theta_k, l_1,\dots,l_k)\):

1. Generate a random number \(\eta\) from Gamma\((a,1)\) distribution;

2. With \(\eta\) as generated in Step 1, generate \(k\)-independent \(X_i\), each following Gamma\((l_i, \eta \theta_i)\), \(i=1,\dots,k\), respectively;

3. To obtain a random sample of size \(n\), repeat the Steps 1 and 2 \(n\) times.

Both algorithms are easily implemented using R stats (R Core Team 2019) functions rexp() and rgamma(), respectively, for generating univariate exponential and gamma random variates. In the following, we describe approaches to generate other distributions related to multivariate Lomax and generalized multivariate Lomax distributions.

Nayak (1987) has also discussed the inter-relationships between many other multivariate distributions and generalized multivariate Lomax distribution. In view of these inter-relationships, the above algorithm can accordingly be amended to simulate data from these distributions - a task which can be quite difficult to accomplish directly. These inter-relationships are described in Table 1. For convenience, we assume \(\mathbf{X} = (X_1,\cdots,X_k)' \sim \text{ML}_k(a;\theta_1,\cdots,\theta_k)\) and \(\mathbf{T} = (T_1,\cdots,T_k)' \sim \text{GML}_k(a;\theta_1,\cdots,\theta_k;l_1,\cdots,l_k)\).

Table 1: Multivariate distributions related to \(\text{GML}_k(a;\theta_1,\cdots,\theta_k;l_1,\cdots,l_k)\).

Multivariate	Transformation/	Probability Density Function
Distribution	Parameter Substitutions
Lomax	\(l_i=1\),	\(f(x_1, \dots, x_k) = \frac{\left[ \prod_{i=1}^{k} \theta_i\right] a(a+1) \cdots (a+k-1)}{\left(1+\sum_{i=1}^{k} \theta_i x_i \right)^{a+k}}\)
	\(i=1,\cdots, k\)	\(x_i > 0, \quad a >0, \quad \theta_i>0\)
	\(l_i=1\),
Mardia’s	\(Y_i=X_i + 1/\theta_i,\)	\(f(y_1, \cdots, y_k) = \frac{\left[ \prod_{i=1}^{k} \theta_i\right] a(a+1) \cdots (a+k-1)}{\left(\sum_{i=1}^{k} \theta_i y_i - k + 1 \right)^{a+k}},\)
Pareto Type I	\(i=1,\cdots, k\)	\(y_i > 1/\theta_i > 0, \quad a >0, \quad \theta_i>0\)
	\(l_i=1\) and \(a=1\),
Logistic	\(W_i=\mu_i-\sigma_i\ln(\theta_iX_i),\)	\(f(w_1, \cdots, w_k) = \frac{k! \exp{\left(-\sum_{i=1}^{k} \frac{w_i - \mu_i}{\sigma_i}\right)}}{\prod_{i=1}^{k} \sigma_i \left(1 + \sum_{i=1}^{k} \exp{(-\frac{w_i - \mu_i}{\sigma_i})}\right)^{1+k}},\)
	\(i=1,\cdots, k\)	\(-\infty<w_i, \mu_i<\infty, \quad \sigma_i > 0\)
	\(l_i=1\),
Burr	\(B_i=(\theta_iX_i/d_i)^{1/c_i},\)	\(f(b_1, \cdots, b_k) = \frac{\left[ \prod_{i=1}^{k} c_i d_i\right] a(a+1) \cdots (a+k-1) \left[ \prod_{i=1}^{k} b_i^{c_i-1}\right]}{\left(1 + \sum_{i=1}^{k} d_i b_i^{c_i}\right)^{a+k}},\)
	\(i=1,\cdots, k\)	\(b_i >0, \quad a>0,\quad c_i>0, \quad d_i>0\)
	\(l_i=1\),
Cook-Johnson’s	\(V_i = (1+\theta_iX_i)^{-a},\)	\(f(v_1, \cdots, v_k) = \frac{\Gamma(a+k)}{\Gamma(a)a^k}\prod_{i=1}^{k} v_i^{(-1/a)-1}\left[\sum_{i=1}^{k} v_i^{-1/a} - k +1\right]^{-(a+k)},\)
uniform	\(i=1,\cdots, k\)	\(0<v_i\le 1, \quad a>0\)
\(F\)
with degrees of freedom	\(\theta_i=l_i/a,\)	\(f(t_1, \cdots, t_k) = \frac{\left[ \prod_{i=1}^{k} (l_i/a)^{l_i}\right] \Gamma\left(\sum_{i=1}^{k} l_i + a \right) \prod_{i=1}^{k} t_i^{l_i-1}}{\Gamma(a) \left[ \prod_{i=1}^{k} \Gamma(l_i) \right] \left(1+\sum_{i=1}^{k} \frac{l_i}{a}t_i \right)^{\sum_{i=1}^{k} l_i + a}},\)
\((2a, 2l_1,\dots,2l_k)\)	\(i=1,\cdots,k\)	\(t_i>0, \quad a>0, \quad l_i>0\)
Inverted Beta	\(\theta_i=1,\)	\(f(t_1, \cdots, t_k) = \frac{\Gamma\left(\sum_{i=1}^{k} l_i + a \right) \prod_{i=1}^{k} t_i^{l_i-1}}{\Gamma(a) \left[ \prod_{i=1}^{k} \Gamma(l_i) \right] \left(1+\sum_{i=1}^{k} t_i \right)^{\sum_{i=1}^{k} l_i + a}},\)
	\(i=1,\cdots,k\)	\(t_i>0, \quad a>0, \quad l_i>0\)

The multivariate \(F\) distribution can also be obtained by considering \[\begin{aligned} T_i = \frac{S_i/(2l_i)}{S_0/(2a)}, \quad i=1,\dots,k, \label{Eq:ChiRatio} \end{aligned} \tag{4}\] where \(S_0, S_1,\dots, S_k\) are independent Chi-square variables with \(2a, 2l_1,\dots,2l_k\) degrees of freedom respectively; see Johnson and Kotz (1972). It is the joint distribution of the ratios of mean squares under certain linear hypotheses on treatments as discussed in Krishnaiah (1965) in the context of simultaneous ANOVA and MANOVA tests where \(S_0\) is a residual sum of squares and \(S_i\)’s are various effect sums of squares. The density given in Table 1 is a special case of generalized multivariate \(F\) distribution defined by Krishnaiah (1965) when \(S_i\)’s, \(i=1,\cdots, k,\) are all independent. This fact is useful in that the Tables given by Armitage and Krishnaiah (1964) make use of this in constructing the statistical tables for certain linear hypotheses. We use these tables to confirm our calculation as done by our R programs. The multivariate Inverted Beta distribution also called the multivariate inverted Dirichlet distribution, is essentially a special case of multivariate \(F\) distribution when \(l_1=l_2=\cdots=l_k=a\).

Figure 1 pictorially summarizes the relationships between (generalized) Lomax and other distributions.

Figure 1: Relationships among (generalized) multivariate Lomax and other distributions. Solid lines represent transformations and dashed dot lines represent reparameterization (parameter substitutions).

3 Probability computations

Here, we give details of computations of cumulative probability distribution function (\(cdf\)), survival function, equicoordinate quantile function for each distribution introduced in Section 2. Depending on the situation, the calculation may sometimes be simpler for joint \(cdf\) or joint survival function.

Distributions transformable from Lomax distribution

The multivariate Lomax distribution has an explicit closed-form expression for the joint survival function given by ((2)). The survival or cumulative distribution function of other related distributions can be obtained either directly or through appropriate transformations. We summarize these explicit expressions of cumulative distribution function \(F(\cdot)\) and survival function \(S(\cdot)\) in Table 2.

Table 2: Cumulative distribution functions and survival functions of multivariate Lomax and related distributions.

Multivariate Distribution	Cumulative Distribution Function	Survival Function
Lomax	No closed form	\(S(x_1, \cdots, x_k) = \left( 1+\sum_{i=1}^{k} \theta_i x_i \right)^{-a}\)
		\(x_i>0, \quad a, \theta_i>0\)
Mardia’s Pareto Type I	No closed form	\(S(y_1, \cdots, y_k) = \left( \sum_{i=1}^{k} \theta_i y_i - k + 1 \right)^{-a}\)
		\(y_i>0, \quad a, \theta_i>0\)
Logistic	\(F(w_1, \cdots, w_k) = \left[ 1 + \sum_{i=1}^{k} \exp(-\frac{w_i-\mu_i}{\sigma_i}) \right]^{-1}\)	No closed form
	\(-\infty < w_i, \mu_i < \infty, \quad \sigma_i>0\)
Burr	No closed form	\(S(b_1,\cdots, b_k) = \left( 1+\sum_{i=1}^{k} d_i b_i^{c_i} \right)^{-a}\)
		\(b_i>0, \quad a, d_i, c_i>0\)
Cook-Johnson’s Uniform	\(F(v_1, \cdots, v_k) = \left[ \sum_{i=1}^{k} v_i^{-1/a} - k + 1 \right]^{-a}\)	No closed form
	\(0<v_i\le 1, \quad a >0\)

For the cumulative distribution functions or survival functions with no closed-form expressions, we rely on the following useful formulas (Joe 1997): \[\begin{aligned} S(\mathbf{x}) = 1 + \sum_{C \in \mathcal{C}} (-1)^{|C|} F_C(x_j, j \in C), \label{eq:cdf} \end{aligned} \tag{5}\]

\[\begin{aligned} F(\mathbf{x}) = 1 + \sum_{C \in \mathcal{C}} (-1)^{|C|} S_C(x_j, j \in C) \label{eq:sf}, \end{aligned} \tag{6}\] where \(F_C(x_j, j \in C)\) (\(S_C(x_j, j \in C)\)) is the joint \(cdf\) (joint survival function) of \(x_j\) where the subscripts belong to the set \(C\), which is a subset of \(\{1, 2, \cdots, k\}\). Clearly, \(C \in \mathcal{C}\) where \(\mathcal{C}\) is the powerset of \(\{1, 2, \cdots, k\}\). Also, \(|C|\) represents the cardinality of \(C\).

The equation \[\begin{aligned} P[x_i \le q_i, i=1,\cdots,k] = \int_{0}^{q_1} \cdots \int_{0}^{q_k} f(x_1, \cdots, x_k) dx_k \cdots dx_1 = p \end{aligned}\] for a given \(p\) does not have a unique solution \((q_1, \cdots, q_k)\). We thus provide the quantile computations only for the equicoordinate quantile, obtained by solving the following equation for \(q\), \[\begin{aligned} P[X_i \le q, i=1,\cdots, k]= \int_{0}^{q} \cdots \int_{0}^{q} f(x_1, \cdots, x_k) dx_k \cdots dx_1 = p, \label{Eq:rootQuan} \end{aligned} \tag{7}\] where \(0 < p < 1\) is a (given) cumulative probability. We make use of the R stats function uniroot(), which is used for finding one dimensional root.

Distributions related to generalized multivariate Lomax distribution

For generalized multivariate Lomax distribution and its related distributions, explicit expressions of the cumulative distribution function and survival function are not available. Thus, we obtain the cumulative probabilities through multiple integral in ((8)) below over the unit cube \([0,1]^k\) by using the adaptive multivariate integration function hcubature() in package cubature (Narasimhan et al. 2018). \[\begin{aligned} \label{Eq:mvInt} F(x_1, \dots, x_k) & = \int_{0}^{x_1} \cdots \int_{0}^{x_k} f(t_1, \cdots, t_k) dt_k \cdots dt_1 \nonumber, \\ & = \prod_{i=1}^k x_i \int_{0}^{1} \cdots \int_{0}^{1} f(u_1x_1, \cdots,u_kx_k) du_k \cdots du_1, \quad (u_i = t_i/x_i, i=1,\cdots, k). \end{aligned} \tag{8}\]

The following result is used for the computation of the cumulative distribution function for the generalized Lomax distribution.
Property 3.1: Let \(T_1, \cdots, T_k\) be \(k\) continuous random variables that jointly follow the GML\(_k(a;\theta_1,\cdots,\theta_k,\)\(l_1,\cdots,l_k)\) distribution as given in ((3)). Then, the cumulative distribution function of \(T_1, \cdots, T_k\) can be computed as \[\begin{aligned} F(x_1, \cdots, x_k) = P(T_1 \le x_1, \cdots, T_k \le x_k) = P(U_1 \le 1, \cdots, U_k \le 1), \end{aligned}\] where \(U_1, \cdots, U_k\) jointly follow GML\(_k(a;\theta_1 x_1,\cdots,\theta_k x_k,l_1,\cdots,l_k)\) distribution.

Proof of the above is straightforward by making the substitutions \(u_i = t_i/x_i\), \(i=1,\cdots,k\).

Through parameter substitutions, the cumulative distribution functions of multivariate \(F\) and the inverted beta distribution can be found. These are summarized in Table 3.

Table 3: Cumulative distribution functions of related multivariate distributions to generalized multivariate Lomax.

Distribution	Parameter	Cumulative Distribution Function
	Substitutions
Multivariate \(F\)
with degrees of freedom	\(\theta_i=l_i/a,\)	\(F(x_1, \cdots, x_k) = P(U_1 \le 1, \cdots, U_k \le 1)\),
\((2a, 2l_1,\dots,2l_k)\)	\(i=1,\cdots,k\)	\((U_1, \cdots, U_k)\) follows GML\(_k(a;x_1 l_1/a,\cdots, x_k l_k/a,l_1,\cdots,l_k)\)
Multivariate	\(\theta_i=1,\)	\(F(x_1, \cdots, x_k) = P(U_1 \le 1, \cdots, U_k \le 1)\),
Inverted Beta	\(i=1,\cdots,k\)	\((U_1, \cdots, U_k)\) follows GML\(_k(a;x_1,\cdots, x_k,l_1,\cdots,l_k)\)

For the above method, the run-time consumption rapidly increases as \(k\) becomes large. Thus as an alternative, we also provide the option of computation of cumulative distribution function via Monte Carlo method. The corresponding algorithm is

Algorithm - CDF Computation using Monte Carlo Method:

1. Generate \(N\) random vectors \(\boldsymbol{t}^{(i)}=(t_1^{(i)}, \cdots, t_k^{(i)})', i = 1,\cdots, N\) from the desired distribution;

2. Compute \[\begin{aligned} \hat{P}(T_1 \le x_1, \cdots, T_k \le x_k) = \frac{1}{N} \sum_{i=1}^{N} \mathcal{I} (\boldsymbol{t}^{(i)} \le \boldsymbol{x}), \end{aligned}\] where \(\boldsymbol{x}=(x_1, \cdots, x_k)'\) and \(\mathcal{I}(\cdot)\) is the zero-one indicator function corresponding to the conditions specified.

Step 1 above is readily carried out by the random numbers generation as described in Section 2 using the package NonNorMvtDist. Since \(cdf\) is computable using adaptive multivariate integration over unit cube \([0,1]^k\) or via the Monte Carlo method, it follows that the survival function can also be calculated (by using ((6))). The equicoordinate quantile is computed by using ((7)).

We also add in our package the calculations of joint probability density function - Being self-explanatory with all \(pdfs\) available in closed form, it needs no further elaboration. The corresponding function is dmv*().

4 Illustrations of simulations and probability calculations

We will illustrate here the functions and corresponding arguments for NonNorMvtDist. The calling sequences include probability density calculation (dmv*), cumulative distribution calculation (pmv*), equicoordinate quantile calculation (qmv*), random numbers generation (rmv*), and survival function calculation (smv*) for each of the multivariate distributions introduced in Section 2. For each distribution, we consider the bivariate case \((k=2)\). This choice enables us to also succinctly and graphically present the probability density plots. The detailed description of the calling sequence for each of the several cases has been moved into a digital complement of this paper.

For example, for the bivariate Lomax distribution \((k=2)\) with parameters \(a=5\), \(\boldsymbol{\theta}=(0.5,1)\), the calling sequences for various functions are

dmvlomax(x, parm1 = 5, parm2 = (0.5,1))
pmvlomax(q, parm1 = 5, parm2 = (0.5,1))
qmvlomax(p, parm1 = 5, parm2 = (0.5,1))
rmvlomax(n, parm1 = 5, parm2 = (0.5,1))
smvlomax(q, parm1 = 5, parm2 = (0.5,1))

It may be mentioned that our approach may be more efficient than the NORTA method (Ghosh and Henderson 2002) for simulation in that NORTA always first requires simulation from multivariate normal, which are then transformed to multivariate uniform. Only often this step, one could subsequently transform the simulated data to the desired distribution. Consequently, for large dimensions, the approach requires more computing power and time (in fact, to simulate data from the normal distribution, many programs themselves first require random number generations from uniform distributions, from which normal random numbers are obtained).

3D bivariate density plot

For each bivariate distribution, we provide the density surface plots along with contours using the function persp3D() from the package plot3D (Soetaert 2017). To illustrate, we define function dplot2(), and we pass appropriate density functions to the density function argument dfun to create the density surfaces. These are summarized in the digital complement along with corresponding resulting plots. For the Lomax distribution with parameters indicated previously, the statements will be

library(plot3D)

dplot2 <- function(dfun, x1, x2, zlim) {
    zmat <- matrix(0, nrow = length(x1), ncol = length(x2))
    for (i in 1:length(x1)) {
        for (j in 1:length(x2)) {
            zmat[i, j] = dfun(x = c(x1[i], x2[j])) 
        } 
    }
    persp3D(z = zmat, x = x1, y = x2, theta = -60, phi = 10, ticktype = "detailed",  
            zlim = zlim, contour = list(nlevels = 30, col = "red"), 
            facets = FALSE, image = list(col = "white", side = "zmin"),
            xlab = "X1", ylab="X2", zlab = "Density", expand = 0.5, d = 2) 
}

dplot2(dfun = function(x) dmvlomax(x, parm1 = 5, parm2 = c(0.5, 1)), x1 = seq(0, 4, 0.1), 
       x2 = seq(0, 4, 0.1), zlim = c(-5, 13))

The plot that results is shown in Figure 2.

Figure 2: Density surface of bivariate Lomax distribution with parameters \(a=5\), \(\boldsymbol{\theta}=(0.5,1)\).

Random number generation

The following code illustrates the use of the function rmvlomax*() with a bivariate sample of size \(n=2\). Sampling is done by setting set.seed(2019) in advance. The digital complement explicitly provides the code as well as output for all of the probability distributions discussed here. In the output, each row represents a bivariate observation.

• Bivariate Lomax: \(a=5\), \(\theta_1=0.5\), \(\theta_2=1\)

  > set.seed(2019)
  > rmvlomax(n = 2, parm1 = 5, parm2 = c(0.5, 1))
        [,1]      [,2]
  [1,] 1.0174406 0.7076480
  [2,] 0.3686253 0.7826978

CDF, survival function and equicoordinate quantile

The applications of \(cdf\) pmv*(), survival function smv*(), and equicoordinate quantile function qmv*() are straightforward and follow the same pattern earlier. See the digital complement for computation details. In the following, we give code as well as output only for Lomax distribution (\(a=5\), \(\theta_1=0.5\), \(\theta_2=1\)) for specified coordinates \((x_1,x_2)\) and for the cumulative probability \(p=0.5\).

• Bivariate Lomax: \(a=5\), \(\theta_1=0.5\), \(\theta_2=1\); quantiles: \((x_1, x_2)=(1, 0.5)\).

      > pmvlomax(q = c(1, 0.5), parm1 = 5, parm2 = c(0.5, 1))
      [1] 0.7678755
      > smvlomax(q = c(1, 0.5), parm1 = 5, parm2 = c(0.5, 1))
      [1] 0.03125
      > qmvlomax(p = 0.5, parm1 = 5, parm2 = c(0.5, 1))
      [1] 0.3928917

5 Computation times

We assess the run-times for the computation of probability (pmv*), equicoordinate quantile function (qmv*) for multivariate Lomax, and generalized multivariate Lomax distributions for reference. The survival function (smv*) of multivariate Lomax distribution has a closed-form expression, and hence the assessment of computation time is omitted. We have used the computer with Intel Core i5-8250U CPU and 8.00 GB RAM. The results for \(p\)-variate Lomax distribution are summarized in Table 4. As we can see, the run-times for pmvlomax() are quit short, even for the dimension \(p=20\). However, qmvlomax() requires a considerable longer time when \(p \ge 17\), which seems to double for every extra dimension added to the size of the random vector. The computation times in Table 4 can also be used as a reference for the distributions related to multivariate Lomax, which are in Table 2 since we apply the same approach for probability computations there as well.

Table 4: Runtimes (in seconds) for functions pmvlomax() and qmvlomax() as functions of \(p\).

\(p\)	pmvlomax()	qmvlomax()		\(p\)	pmvlomax()	qmvlomax()
1	0.01695895	0.03702188		11	0.04889703	0.7992568
2	0.01795197	0.02293706		12	0.09025002	1.879766
3	0.01794696	0.02789807		13	0.1545889	3.087925
4	0.01795197	0.03290296		14	0.245369	5.950396
5	0.01976085	0.04189205		15	0.4657819	11.37162
6	0.01995182	0.06582808		16	0.853502	22.04591
7	0.02094102	0.098768		17	1.708418	45.51909
8	0.02197218	0.146611		18	2.803703	90.70764
9	0.02396512	0.2393882		19	5.453189	181.55772
10	0.04587412	0.4296861		20	10.40903	351.58896

The results for generalized \(p\)-variate Lomax distribution are summarized in Tables 5 and 6. Both functions pmvglomax() and smvglomax() require relatively much longer time when \(p > 4\), and qmvglomax() takes longer when \(p > 2\). Based on the run-time consumption, we recommend algorithm MC for larger dimensions (e.g., when \(p>5\)). Similarly, this run-time study can also be used as a reference for the related distributions (to generalized Lomax distribution) as listed in Table 3 since we apply the same method for computations.

Table 5: Runtimes (in seconds) for functions pmvglomax() and smvglomax() by using algorithms numerical and MC as functions of \(p\).

	pmvglomax()			smvglomax()
\(p\)	numerical	MC		numerical	MC
1	0.03395414	4.600386		0.03084397	3.977374
2	0.02397585	6.536522		0.05378199	5.510087
3	0.101758	8.396577		0.208086	7.229721
4	0.9758801	10.64278		2.13447	8.741386
5	16.43411	19.30024		40.11997	9.762063
6	305.50092	19.57221		1344.1908	11.63218

Table 6: Runtimes (in seconds) for function qmvglomax() by using algorithms numerical and MC as functions of \(p\).

	qmvglomax()
\(p\)	numerical	MC
1	0.4144461	12.5594
2	6.383504	18.97528
3	96.86238	27.23917
4	2929.5018	30.62036

6 Maximum likelihood estimation of parameters

We also include the maximum likelihood estimation to estimate the parameters for various Lomax related distributions (except for the bivariate \(F\) distribution). Although many of the density functions in Section 2 have complicated forms, maximum likelihood estimation can be easily accomplished by using the built-in optimization functions in R stats. The log-likelihood function for a given sample \(\mathbf{x_1, \dots, x_n}\) is given by \[\begin{aligned} \label{LLK} \mathcal{L}(\boldsymbol{\theta}|\mathbf{x_1, \dots, x_n}) &= \log \left[ \prod_{j=1}^{n} f(x_{j1}, \dots, x_{jk}|\boldsymbol{\theta}) \right] \nonumber\\ &= \sum_{j=1}^{n} \log \left[ f(x_{j1}, \dots, x_{jk}|\boldsymbol{\theta}) \right], \qquad \boldsymbol{\theta} \in \Theta, \end{aligned} \tag{9}\] where \(n\) is the sample size, \(\boldsymbol{\theta}\) is a vector of parameters to be estimated, and \(\mathbf{x}_j = (x_{j1}, \dots, x_{jk})'\) is the \(j\)th observation for the random vector \(\mathbf{X} =(X_1,\dots, X_k)'\), respectively. The maximizer \(\hat{ \boldsymbol{\theta}}\) of the log-likelihood function given in ((9)), namely, \[\begin{aligned} \hat{\boldsymbol{\theta}} = \arg \max_{\boldsymbol{\theta} \in \Theta} \mathcal{L}(\boldsymbol{\theta}|\mathbf{x_1, \dots, x_n}), \end{aligned}\] is obtained using an appropriate optimization method. The parameter space \(\Theta\) in each case must be appropriately constrained, and these constraints must be taken into account during the optimization process. We have thus made use of three R stats functions, namely, optim(), constrOptim(), and optimize() in this work. The functionality of these optimization functions is described in Table 7.

Table 7: Use of optimization functions in R stats.

Function	Number of parameters	Usage
optim()	Multiple	General-purpose Optimization
constrOptim()	Multiple	Optimization with linear constraints
optimize()	Single	One Dimensional Optimization

By default, all these functions perform the task of minimization of a function. To maximize ((9)), we only need to add argument control = list(fnscale = -1) in functions optim() and constrOptim(), and set maximum = TRUE in function optimize().

For example, for the multivariate Lomax distribution, we define the log-likelihood function loglik.lomax() by using the following code:

loglik.lomax <- function(data, par) {
    ll <- sum(dmvlomax(data, parm1 = par[1], parm2 = par[-1], log = TRUE))
}

The R stats function constrOptim() is chosen to obtain the maximizer of the log-likelihood function (or equivalently, loglik.lomax()). The linear constraints imposed on the parameters \(a, \theta_1\), and \(\theta_2\) are \(a > 0\), \(\theta_1 > 0\), and \(\theta_2 > 0\). In matrix notation, it is, \[\begin{aligned} \boldsymbol{U} \boldsymbol{\theta} = \begin{pmatrix} 1 & 0 & 0 \\ 0 & 1 & 0 \\ 0 & 0 & 1 \end{pmatrix} \begin{pmatrix} a \\ \theta_1 \\ \theta_2 \end{pmatrix} = \begin{pmatrix} a \\ \theta_1 \\ \theta_2 \end{pmatrix} > \begin{pmatrix} 0 \\ 0 \\ 0 \end{pmatrix} = \boldsymbol{C}. \end{aligned}\] Thus, in the code that follows, the constraint matrix ui is set as an identity matrix \(\mathbf{I}_3\) by using the function diag(3), and constraint vector ci is set as a zero vector rep(0, 3). For our illustration, let the data be the one simulated by the methods described earlier, each with \(n=300\). It is done by

set.seed(1)
bvtlomax <- rmvlomax(n = 300, parm1 = 5, parm2 = c(0.5, 1))

The starting initial values for \(a, \theta_1\), and \(\theta_2\) are all set as 10 by assigning rep(10, 3) to the argument theta in function constrOptim(). The gradient argument grad is optional, and we have chosen this to be NULL.

> est = constrOptim(theta = rep(10, 3), f = loglik.lomax, grad = NULL, data = bvtlomax, 
+                   ui = diag(3), ci = rep(0, 3), control = list(fnscale = -1))
> est$convergence
[1] 0
> est$par
[1] 5.0555691 0.4468724 0.9036692

The output consists of two important pieces of information, (i) whether convergence is successfully achieved (est$convergence = 0) or not (est$convergence = 1) and (ii) the values of the final maximum likelihood estimates. In our illustration, the convergence is successfully achieved, and we have \(\hat{a} = 5.0555691\) and \((\hat{\theta}_1, \hat{\theta}_2)=(0.4468724, 0.9036692)\).

We summarize the optimization methods, constraints, and the resulting outputs for all the bivariate distributions (except for bivariate \(F\) distribution) in Table 8. The detailed illustrations and codes for the remaining distributions are included in the digital complement. Observe that these estimates are reasonably close to the true parameter values, thereby confirming that the program is functioning as it is expected to.

Table 8: Comparison between True and Estimated Values of Parameters for each of the Distributions (except for bivariate \(F\) distribution).

Multivariate	Parameters	Optimization	Constraints	Estimated Parameters
Distribution		Method
Lomax	\(a=5\)	constrOptim()	\(U=I_3\)	\(\hat{a} = 5.05556\)
	\(\boldsymbol{\theta}=(0.5, 1)'\)		\(C=(0,0,0)'\)	\(\hat{\boldsymbol{\theta}}=(0.44687, 0.90366)'\)
Mardia’s	\(a=5\)	constrOptim()	\(U=I_3\)	\(\hat{a} = 4.63862\)
Pareto Type I	\(\boldsymbol{\theta}=(0.5,2)'\)		\(C=(0, [\min(X_1)]^{-1}, [\min(X_2)]^{-1})'\)	\(\hat{\boldsymbol{\theta}}=(0.49971, 1.99785)'\)
Logistic	\(\boldsymbol{\mu}=(0.5, 1)'\)	optim()	N/A	\(\hat{\boldsymbol{\mu}} = (0.39199, 0.89731)'\)
	\(\boldsymbol{\sigma}=(1,1.5)'\)			\(\hat{\boldsymbol{\sigma}} = (0.97573, 1.55976)'\)
Burr	\(a=3\)	constrOptim()	\(U=I_5\)	\(\hat{a} = 3.91498\)
	\(\boldsymbol{d}=(1,3)'\)		\(C=(0,0,0,0,0)'\)	\(\hat{\boldsymbol{d}}=(0.64889, 2.06858)'\)
	\(\boldsymbol{c}=(2,5)'\)			\(\hat{\boldsymbol{c}}=(1.92719, 5.07697)'\)
Cook-Johnson’s Uniform	\(a=0.3\)	optimize()	\(a > 0\)	\(\hat{a} = 0.31064\)
Generalized	\(a=5\)	constrOptim()	\(U=I_5\)	\(\hat{a} = 3.91869\)
Lomax	\(\boldsymbol{\theta}=(0.5, 1)'\)		\(C=(0,0,0,0,0)'\)	\(\hat{\boldsymbol{\theta}} = (0.54674, 1.79126)'\)
	\(\boldsymbol{l}=(2, 4)'\)			\(\hat{\boldsymbol{l}} = (1.70310, 5.21736)'\)
Inverted Beta	\(a=4\)	constrOptim()	\(U=I_3\)	\(\hat{a} = 3.67153\)
	\(\boldsymbol{l}=(2, 6)'\)		\(C=(0,0,0)'\)	\(\hat{\boldsymbol{l}} = (1.82450, 5.46465)'\)

7 Two applications

In this last section, we give two brief applications, which not only demonstrate the use but also confirm the accuracy and verify the correctness of our work.

Generating data from the nonelliptical symmetric distributions with univariate normal marginals

Cook-Johnson’s multivariate uniform distribution is a family of distributions that can be used for modeling nonelliptical symmetric data. Further, in view of uniform distribution for marginal, it has been as one of the useful choices for modeling through copula (in fact, Cook-Johnson’s uniform distribution is indeed a Clayton copula (Nelsen 2006)). The value of parameter \(a\) impacts the common correlation coefficient \(\rho\) among variates in that \(\rho \rightarrow 0\) as \(a \rightarrow \infty\), and \(\rho \rightarrow 1\) as \(a \rightarrow 0\) (Cook and Johnson 1981). An interesting application of Cook-Johnson’s multivariate uniform distribution is to obtain new joint distributions by marginal transformation. Specifically, we consider the problem of generating random numbers from a multivariate distribution that is not elliptically symmetric but has univariate normal marginals. Let \(U_i\)’s, \(i=1,2\), be two random variables corresponding to the Cook-Johnson’s bivariate uniform distribution. The following code yields the pairs of random numbers, each having the normal marginals by the transformation \(X_i = \Phi^{-1}(Ui)\), where \(\Phi^{-1}(\cdot)\) is the quantile function of a standard normal distribution. Clearly, the joint distribution of \(X_1\) and \(X_2\) is not bivariate normal. To begin with, the parameter \(a\) is taken to be \(a=2\).

set.seed(1)
biv.unif <- rmvunif(8000, parm = 2, dim = 2)
biv.norm <- as.data.frame(apply(biv.unif, 2, qnorm))

The sample correlation coefficient \(\rho\) of data set biv.norm is computed by the following code.

> cor(biv.norm$V1, biv.norm$V2)
[1] 0.3180119

We create a bivariate scatter plot using the function ggplot() in package ggplot2 (Wickham 2016) for data set biv.norm. This is shown in Figure 3 (a).

library(ggplot2)

ggplot(biv.norm, aes(x = V1, y = V2)) + xlim(c(-4, 4)) + 
ylim(c(-4, 4)) + xlab("X1") + ylab("X2") + geom_point()

To assess the behavior as a function of \(a\), we now decrease the parameter \(a\) to \(1.0, 0.5, 0.1\) resulting in higher correlations \(\rho\) (\(=0.51, 0.68, 0.93\), respectively) between the two variates. The bivariate scatter plots for the four cases that is, when \(a = 2.0, 1.0, 0.5, 0.1\) are shown in Figure 3 (a)-(d). It is easy to observe that the generated bivariate data have nonelliptical yet, symmetric contours.

a = 2, ρ = 0.32	a = 1, ρ = 0.51
a = 0.5, ρ = 0.68	a = 0.1, ρ = 0.93

Figure 3: The scatterplots of nonelliptical symmetric normal data generated from transformed Cook-Johnson’s uniform random numbers with a = 2, 1, 0.5 and 0.1.

Creating tables for simultaneous MANOVA hypothesis tests

Multivariate \(F\) distribution arises naturally as the distribution of test statistics in several testing problems in simultaneous MANOVA. Let \(s_1^2, \cdots, s_k^2\) be \(k\) independent sums of squares, and \(s_0^2\) be the sum of squares due to error in the classical ANOVA model. Also, let \(H_1, \cdots, H_k\) be certain individual linear hypotheses with the corresponding sum of squares \(s_1^2, \cdots, s_k^2\). Assume that under \(H_1,\cdots, H_k\), respectively, the sums of squares \(s_1^2, \cdots, s_k^2\) are all \(\chi^2\) random variables each with \(n\) degrees of freedom and \(s_0^2\) is a \(\chi^2\) random variable with \(m\) degrees of freedom and is independent of \(s_1^2, \cdots, s_k^2\). Armitage and Krishnaiah (1964) defined the critical values \(F_{\alpha}\) at \(\alpha\) level of significance for simultaneously testing hypotheses \(H_1, \cdots, H_k\) by the probability statement, \[\begin{aligned} P \left[ \frac{m \max{(s_1^2, \cdots, s_k^2)}}{n s_0^2} \le F_{\alpha} \Bigg| \bigcap_{i=1}^k H_i \right] = P \left[ \frac{m s_i^2}{n s_0^2} \le F_{\alpha} , i=1, \cdots, k \Bigg| \bigcap_{i=1}^k H_i \right] = 1 - \alpha. \end{aligned}\] The quantities \((m s_i^2)/(n s_0^2), i=1, \cdots, k\) jointly follow multivariate \(F\) distribution if the overall null hypothesis \(H_0=\bigcap_{i=1}^k H_i\) is true. In this case, the critical value \(F_{\alpha}\) can be readily computed using the equicoordinate quantile function qmvf() by setting the argument corresponding to \(k+1\) values of the degrees of freedom as df = c(m, n, ..., n). The following code gives \(F_{0.05}=9.551505\) for the bivariate \(F\) case when \(m=5\) and \(n=1\) with default algorithm by using adaptive multiple integration over unit cube (algorithm = "numerical"). With Monte Carlo algorithm (algorithm = "MC" with nsim=1,000,000), we obtain \(F_{0.05}=9.550944\). Note that the Monte Carlo method is seed dependent, so the output from different runs may slightly differ from each other.

> qmvf(0.95, df = c(5, 1, 1))
[1] 9.551505
> qmvf(0.95, df = c(5, 1, 1), algorithm = "MC")
[1] 9.550944

For further demonstration and also to further affirm our trust in the calculations, we compare the output of quantile function qmvf() using both adaptive multivariate integration and Monte Carlo methods with the values given in Armitage and Krishnaiah (1964). These three calculations are reported in Table 9 for a few choices of \(m\) and \(n\). The agreement among the three columns shows that the package NonNorMvtDist provides a convenient way to obtain percentage points for the hypothesis testing problems considered by Armitage and Krishnaiah (1964) and Krishnaiah (1965). Clearly, unlike the tables in Armitage and Krishnaiah, the choices of \(\alpha\) and degrees of freedom are not restricted, and in that sense, our package is very comprehensive and exhaustive in this respect.

Table 9: Comparison between the output of qmvf() with the values given in Armitage and Krishnaiah (1964).

\(\alpha\)	df \((m,n,n)\)	qmvf() Output	qmvf() Output	Tabulated Values in
		(algorithm = "numerical")	(algorithm = "MC")	Armitage and Krishnaiah (1964, pp. 33-42)
\(0.05\)	\((5,1,1)\)	9.551505	9.550944	9.55
	\((5,2,2)\)	7.879999	7.881698	7.88
	\((5,3,3)\)	7.136473	7.165361	7.14
	\((5,4,4)\)	6.702224	6.715759	6.70
	\((5,5,5)\)	6.412372	6.399167	6.41
	\((10,6,6)\)	3.899335	3.898442	3.90
	\((10,7,7)\)	3.768494	3.767915	3.77
	\((10,8,8)\)	3.665646	3.661366	3.67
	\((10,9,9)\)	3.582271	3.583923	3.58
	\((10,10,10)\)	3.513163	3.514059	3.51

8 Concluding remarks

We have developed a new R package, NonNorMvtDist, for generating multivariate random numbers from Lomax (Pareto type II), generalized Lomax, Mardia’s Pareto type I, logistic, Burr, Cook-Johnson’s uniform, \(F\), and inverted beta distributions. Detailed examples of each distribution are given to illustrate data simulation, probability calculations, and statistical modeling.

The fact that nonnormal and skewed multivariate distributions are common in the real world but are rarely pursued for analysis due to the lack of ready-to-use computational support underscores the importance of this package. Possibilities of the use of these distributions are practically limitless and yet unforeseen in a variety of areas, starting from the biomedical sciences, reliability, and engineering as well as in statistical finance in the contexts of volatility estimation. Simulations, probability calculations, as well as calculations of quantiles, and the maximum likelihood estimation of parameters are the natural first set of computations in such studies. We have addressed all of these in this work.

The calculations of probabilities of hypercubes (for example, of \(P[a_1 < X_1 < b_1, a_2 < X_2 < b_2, a_3 < X_3 < b_3]\)) can be easily implemented by appropriately combining several \(cdf\) calculations. Alternatively, our codes for pmv*() can be suitably modified for this purpose. The probability density surface plots for \(any\) bivariate marginal can be easily constructed since, for the multivariate Lomax distribution, the marginal distributions of any subset of random variables also follow the multivariate Lomax distribution in the appropriate dimension. Further, our work provides a way to generate data from, probability calculations for, as well as modeling for, the data which are marginally distributed as normal but jointly are not.

9 Acknowledgements

Content of this article constitutes a part of the PhD dissertation of Zhixin Lun at Oakland University. Financial research support from Oakland University during the summer of Year 2019 is acknowledged. We would like to thank the anonymous referees for their helpful comments and remarks to improve both package and this paper.

CRAN packages used

NonNorMvtDist, VGAM, stats, cubature, plot3D, ggplot2

CRAN Task Views implied by cited packages

Distributions, Econometrics, Environmetrics, ExtremeValue, NumericalMathematics, Phylogenetics, Psychometrics, Spatial, Survival, TeachingStatistics

Note

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