#' Probability Reference Sample 2 (sp2)
#'
#' This dataset represents a probability survey derived from the
#' 1997--2008 cycles of the National Health Interview Survey (NHIS).
#' It is intended for use
#' alongside \code{sc} and \code{sp1} to illustrate the multi-reference
#' calibration method implemented in the \code{nonprobsampling} package.
#'
#' @format A data frame with 35525 observations and 11 variables:
#' \describe{
#'   \item{agecat}{Age category (factor with 4 levels: 1 = 55--59, 2 = 60--64, 3 = 65--69, 4 = 70+)}
#'   \item{marital}{Marital status (factor with 4 levels: 1 = Married Or Living As Married, 2 = Widowed, 3 = Divorced or Separated, 4 = Never Married)}
#'   \item{race}{Race category (factor with 4 levels: 1 = White, 2 = Black, 3 = Hispanic, 4 = Other)}
#'   \item{employment}{Employment status (factor with 2 levels: 0 = Not Working, 1 = Working)}
#'   \item{diabetes}{Diabetes diagnosis indicator (factor with 2 levels: 0 = No, 1 = Yes)}
#'   \item{BMI}{Body mass index category (factor with 4 levels: "Normal", "Overweight", "Obese", "Morbidly Obese")}
#'   \item{smoking}{Smoking status (factor with 3 levels: 1 = Never Smoker, 2 = Former Smoker, 3 = Current Smoker)}
#'   \item{comorbidity}{General comorbidity indicator (factor with 2 levels: 0 = No, 1 = Yes)}
#'   \item{wts_sp2}{Sampling weights (numeric)}
#'   \item{strata_sp2}{Stratum identifier for complex survey design (numeric)}
#'   \item{psu_sp2}{Primary sampling unit identifier for complex survey design (numeric)}
#' }
#'
#' @details
#' The dataset includes auxiliary variables shared with the nonprobability
#' sample \code{sc}, enabling the construction of pseudo-weights to adjust
#' for participation bias. Survey design variables and sampling weights are
#' provided to support design-consistent estimation.
#'
#' @source
#' Derived from the National Health Interview Survey (NHIS), 1997--2008 cycles,
#' conducted by the U.S. National Center for Health Statistics (NCHS).
#'
#' @usage data(sp2)
#'
#' @examples
#' data(sp2)
#' str(sp2)
#' summary(sp2)
"sp2"
