grgkw: Gradient of the Negative Log-Likelihood for the GKw...

In gkwreg: Generalized Kumaraswamy Regression Models for Bounded Data

Gradient of the Negative Log-Likelihood for the GKw Distribution

Description

Computes the gradient vector (vector of partial derivatives) of the negative log-likelihood function for the five-parameter Generalized Kumaraswamy (GKw) distribution. This provides the analytical gradient, often used for efficient optimization via maximum likelihood estimation.

Usage

grgkw(par, data)

Arguments

par	A numeric vector of length 5 containing the distribution parameters in the order: alpha (\alpha > 0), beta (\beta > 0), gamma (\gamma > 0), delta (\delta \ge 0), lambda (\lambda > 0).
data	A numeric vector of observations. All values must be strictly between 0 and 1 (exclusive).

Details

The components of the gradient vector of the negative log-likelihood (-\nabla \ell(\theta | \mathbf{x})) are:

-\frac{\partial \ell}{\partial \alpha} = -\frac{n}{\alpha} - \sum_{i=1}^{n}\ln(x_i) + \sum_{i=1}^{n}\left[x_i^{\alpha} \ln(x_i) \left(\frac{\beta-1}{v_i} - \frac{(\gamma\lambda-1) \beta v_i^{\beta-1}}{w_i} + \frac{\delta \lambda \beta v_i^{\beta-1} w_i^{\lambda-1}}{z_i}\right)\right]

-\frac{\partial \ell}{\partial \beta} = -\frac{n}{\beta} - \sum_{i=1}^{n}\ln(v_i) + \sum_{i=1}^{n}\left[v_i^{\beta} \ln(v_i) \left(\frac{\gamma\lambda-1}{w_i} - \frac{\delta \lambda w_i^{\lambda-1}}{z_i}\right)\right]

-\frac{\partial \ell}{\partial \gamma} = n[\psi(\gamma) - \psi(\gamma+\delta+1)] - \lambda\sum_{i=1}^{n}\ln(w_i)

-\frac{\partial \ell}{\partial \delta} = n[\psi(\delta+1) - \psi(\gamma+\delta+1)] - \sum_{i=1}^{n}\ln(z_i)

-\frac{\partial \ell}{\partial \lambda} = -\frac{n}{\lambda} - \gamma\sum_{i=1}^{n}\ln(w_i) + \delta\sum_{i=1}^{n}\frac{w_i^{\lambda}\ln(w_i)}{z_i}

where:

• v_i = 1 - x_i^{\alpha}

• w_i = 1 - v_i^{\beta} = 1 - (1-x_i^{\alpha})^{\beta}

• z_i = 1 - w_i^{\lambda} = 1 - [1-(1-x_i^{\alpha})^{\beta}]^{\lambda}

• \psi(\cdot) is the digamma function (digamma).

Numerical stability is ensured through careful implementation, including checks for valid inputs and handling of intermediate calculations involving potentially small or large numbers, often leveraging the Armadillo C++ library for efficiency.

Value

Returns a numeric vector of length 5 containing the partial derivatives of the negative log-likelihood function -\ell(\theta | \mathbf{x}) with respect to each parameter: (-\partial \ell/\partial \alpha, -\partial \ell/\partial \beta, -\partial \ell/\partial \gamma, -\partial \ell/\partial \delta, -\partial \ell/\partial \lambda). Returns a vector of NaN if any parameter values are invalid according to their constraints, or if any value in data is not in the interval (0, 1).

Author(s)

Lopes, J. E.

References

Cordeiro, G. M., & de Castro, M. (2011). A new family of generalized distributions. Journal of Statistical Computation and Simulation

Kumaraswamy, P. (1980). A generalized probability density function for double-bounded random processes. Journal of Hydrology, 46(1-2), 79-88.

See Also

llgkw (negative log-likelihood), hsgkw (Hessian matrix), dgkw (density), optim, grad (for numerical gradient comparison), digamma

Examples

# Generate sample data from a known GKw distribution
set.seed(123)
true_par <- c(alpha = 2, beta = 3, gamma = 1.0, delta = 0.5, lambda = 0.5)
sample_data <- rgkw(100, alpha = true_par[1], beta = true_par[2],
                   gamma = true_par[3], delta = true_par[4], lambda = true_par[5])

# --- Use in Optimization (e.g., with optim using analytical gradient) ---
start_par <- c(1.5, 2.5, 1.2, 0.3, 0.6) # Initial guess

# Optimization using analytical gradient
mle_result_gr <- stats::optim(par = start_par,
                              fn = llgkw,  # Objective function (Negative LL)
                              gr = grgkw,  # Gradient function
                              method = "BFGS", # Method using gradient
                              hessian = TRUE,
                              data = sample_data)

if (mle_result_gr$convergence == 0) {
  print("Optimization with analytical gradient converged.")
  mle_par_gr <- mle_result_gr$par
  print("Estimated parameters:")
  print(mle_par_gr)
} else {
  warning("Optimization with analytical gradient failed!")
}

# --- Compare analytical gradient to numerical gradient ---
# Requires the 'numDeriv' package
if (requireNamespace("numDeriv", quietly = TRUE) && mle_result_gr$convergence == 0) {

  cat("\nComparing Gradients at MLE estimates:\n")

  # Numerical gradient of the negative log-likelihood function
  num_grad <- numDeriv::grad(func = llgkw, x = mle_par_gr, data = sample_data)

  # Analytical gradient (output of grgkw)
  ana_grad <- grgkw(par = mle_par_gr, data = sample_data)

  cat("Numerical Gradient:\n")
  print(num_grad)
  cat("Analytical Gradient:\n")
  print(ana_grad)

  # Check differences (should be small)
  cat("Max absolute difference between gradients:\n")
  print(max(abs(num_grad - ana_grad)))

} else {
  cat("\nSkipping gradient comparison (requires 'numDeriv' package or convergence).\n")
}

gkwreg documentation built on April 16, 2025, 1:10 a.m.