#' xtfifevd: Panel Fixed Effects Filtered and Variance Decomposition Estimation
#'
#' @description
#' Implements fixed effects estimators for time-invariant variables in panel
#' data models. Standard fixed effects (FE) estimation cannot identify
#' coefficients on time-invariant regressors because they are collinear with

#' the individual fixed effects. This package provides three methods to estimate
#' these coefficients:
#'
#' @section Methods:
#'
#' \describe{
#'   \item{FEVD}{Fixed Effects Vector Decomposition (Plümper & Troeger, 2007).
#'     A three-stage estimator that decomposes the unit effects into explained
#'     (by time-invariant variables) and unexplained components.}
#'   \item{FEF}{Fixed Effects Filtered (Pesaran & Zhou, 2016). A two-stage
#'     estimator that regresses time-averaged FE residuals on time-invariant
#'     variables.}
#'   \item{FEF-IV}{Fixed Effects Filtered with Instrumental Variables
#'     (Pesaran & Zhou, 2016). Uses external instruments when time-invariant
#'     variables are endogenous.}
#' }
#'
#' @section Variance Estimation:
#' All methods use the Pesaran-Zhou (2016) variance estimator (Equation 17 for
#' FEF/FEVD, Equation 51 for FEF-IV). These correct variance estimators account
#' for the generated regressor problem and avoid the severe size distortions
#' documented for naive pooled OLS standard errors.
#'
#' @references
#' Plumper, T., & Troeger, V. E. (2007). Efficient Estimation of Time-Invariant
#' and Rarely Changing Variables in Finite Sample Panel Analyses with Unit Fixed
#' Effects. \emph{Political Analysis}, 15(2), 124-139.
#' \doi{10.1093/pan/mpm002}
#'
#' Pesaran, M. H., & Zhou, Q. (2018). Estimation of time-invariant effects in
#' static panel data models. \emph{Econometric Reviews}, 37(10), 1137-1171.
#' \doi{10.1080/07474938.2016.1222225}
#'
#' Greene, W. H. (2011). Fixed Effects Vector Decomposition: A Magical Solution
#' to the Problem of Time-Invariant Variables in Fixed Effects Models?
#' \emph{Political Analysis}, 19(2), 135-146.
#' \doi{10.1093/pan/mpq034}
#'
#' @docType package
#' @name xtfifevd-package
#' @aliases xtfifevd-package
#' @keywords internal
"_PACKAGE"
