Changes in version 2.0.1 (2025-06-02)                  

Bug Fixes

  - Corrected the initialization of the start values in the HeckmanSK
    function. Previously, it was relying on a two-step method to
    generate starting values, which could lead to numerical instability
    in some cases. Now, a more robust initialization is implemented to
    ensure better convergence and numerical stability.
  - Fixed the display of the log-likelihood in the summary methods of
    all functions (e.g., summary.HeckmanSK, summary.HeckmanCL,
    summary.HeckmanBS, etc.). Previously, these were reporting the
    negative of the log-likelihood. They now correctly display the
    log-likelihood value as returned by the optimization procedure.

                 Changes in version 2.0.0 (2025-06-01)                  

Major updates

  - Complete overhaul of the package, improving organization,
    readability, and performance of all functions.
  - Rewritten log-likelihood and gradient functions (loglik_* and
    gradlik_*) for enhanced numerical stability and clarity.
  - Fixed discrepancies where analytical gradients did not match
    numerical gradients.
  - Comprehensive documentation updates for all functions, ensuring
    better understanding and usage.
  - Added two new helper functions:
      - postprocess_theta(): streamlines parameter transformations for
        clear interpretation and improved consistency across models.
      - extract_model_components(): extracts model.frame, model.matrix,
        and model.response objects in a robust and reusable way.
  - All functions now follow consistent coding style and best practices.
  - Significant performance improvements, making the package lighter and
    more efficient.

Bug fixes

  - Fixed issues with incorrect gradient calculations for sigma and rho
    parameters.
  - Corrected numerical errors in several model functions.

Other improvements

  - Updated vignette and examples to reflect the new structure and
    improvements.
  - Switched pkgdown site to Bootstrap 5 for improved readability and
    responsiveness.

                 Changes in version 1.0.1 (2022-10-04)                  

Minor updates

  - Improved documentation and examples.
  - Added unit tests to ensure stability of HeckmanCL() and other core
    functions.

                        Changes in version 1.0.0                        

Initial release

  - Initial implementation of the classic Heckman model (HeckmanCL())
    and foundational sample selection models.
  - Basic infrastructure for selection bias correction in econometric
    models.