Introduction to the BetaDanish Package

1. Introduction

The BetaDanish package provides a comprehensive suite of tools for survival and reliability analysis using the Beta-Danish distribution.

Classical lifetime models, such as the Weibull, Gamma, or Log-Normal distributions, are often limited in their hazard shape flexibility. They typically force the hazard rate to be strictly monotonic or, at best, unimodal. The Beta-Danish distribution overcomes these limitations by accommodating decreasing, increasing, unimodal (upside-down bathtub), and bathtub-shaped hazard rates within a single, unified four-parameter family.

This package implements the core mathematical functions, robust Maximum Likelihood Estimation (MLE) for complete and right-censored data, and advanced modules for Accelerated Failure Time (AFT) regression, cure-rate models, and competing risks.

2. Theoretical Background

The Beta-Danish distribution is constructed by applying the beta-generated transformation (Eugene et al., 2002) to a baseline distribution. The baseline used here is the two-parameter exponentiated log-logistic distribution.

Let \(a, b, c, k > 0\) be the parameters of the distribution. * \(k\) is the scale parameter. * \(c\) is the baseline shape parameter. * \(a\) and \(b\) are the beta-generator shape parameters controlling skewness and tail weight.

The Cumulative Distribution Function (CDF) is given by the regularized incomplete beta function: \[ F(t) = I_{G(t)}(a, b) \] where \(G(t) = \left( \frac{k t}{1 + k t} \right)^c\).

The Probability Density Function (PDF) is: \[ f(t) = \frac{c k}{B(a, b)} \frac{(k t)^{ca - 1}}{(1 + k t)^{ca + 1}} \left[ 1 - \left( \frac{k t}{1 + k t} \right)^c \right]^{b - 1} \]

3. Core Distribution Functions

The package provides the standard d/p/q/r/h functions. All internal calculations are performed in log-space to ensure numerical stability, especially in the tails.

library(BetaDanish)

# Calculate the hazard rate at time t = 2
hbetadanish(x = 2, a = 1.5, b = 2.0, c = 3.0, k = 0.5)

# Generate 100 random survival times
set.seed(2026)
sim_data <- rbetadanish(n = 100, a = 1.5, b = 2.0, c = 3.0, k = 0.5)

4. Fitting Models to Data

The fit_betadanish() function is the core MLE engine. It uses a multi-start optimization strategy to ensure global convergence and strictly enforces positivity constraints via log-scale reparameterization.

# Load the built-in dataset
data("remission", package = "BetaDanish")

# Fit the full 4-parameter model
fit_full <- fit_betadanish(survival::Surv(time, status) ~ 1, data = remission)
summary(fit_full)

# Fit the 3-parameter submodel (a = 1)
fit_sub <- fit_betadanish(survival::Surv(time, status) ~ 1, data = remission, submodel = TRUE)

# Compare the nested models using a Likelihood Ratio Test (LRT)
compare_models(fit_full, fit_sub)

# Generates Survival, Hazard, Density, P-P, and Q-Q plots
plot(fit_full, type = "all")

data("transplant", package = "BetaDanish")

# Fit the model to censored data
fit_censored <- fit_betadanish(survival::Surv(time, status) ~ 1, data = transplant)
summary(fit_censored)

# Plot the Kaplan-Meier curve overlaid with the Beta-Danish fit
plot(fit_censored, type = "survival")

5. Advanced Modeling: Cure Rates

In many clinical datasets (like the transplant data), a proportion of patients may be “cured” and will never experience the event. Standard survival models force the survival curve to zero, which misrepresents the data.

The BetaDanish package provides the fit_bd_cure() function to fit both Mixture and Promotion-Time (Non-Mixture) cure models.

# Fit a mixture cure model
# Latency (time to event for susceptible) has no covariates (~ 1)
# Incidence (probability of being susceptible) depends on treatment group (~ group)
cure_fit <- fit_bd_cure(
  formula_aft = survival::Surv(time, status) ~ 1,
  formula_cure = ~ group,
  data = transplant,
  type = "mixture"
)

summary(cure_fit)

6. Automated Benchmarking

To justify the use of the Beta-Danish distribution, you can benchmark it against classical distributions (Weibull, Gamma, Log-Normal, etc.) using the compare_distributions() function. (Note: This requires the flexsurv package).

# Returns a ranked table of AIC, BIC, and Log-Likelihoods
compare_distributions(fit_full)

7. Conclusion

The BetaDanish package offers a robust, flexible, and user-friendly environment for advanced survival analysis. By unifying standard MLE fitting, comprehensive diagnostics, and advanced modules (AFT, Cure, Competing Risks) under a single framework, it serves as a powerful tool for statisticians and applied researchers alike.