#' Expanded GDP and Trade Data, compiled by Kristian Skrede Gleditsch, v. 6.0
#'
#' These data provide estimates of GDP per capita of independent states. The
#' core input data used is PWT 8.0. Unlike previous versions of the GDP data
#' this reports PPP estimates of real GDP rather than per capita GDP.
#'
#' @format A data frame with 9627 rows and 8 variables: \describe{
#' \item{statenum}{Numeric country identifier} \item{statid}{Three letter
#' acronym country identifier For a list of the country id and numeric codes,
#' see \url{http://privatewww.essex.ac.uk/~ksg/statelist.html}. Further
#' details are available in case description file and the paper describing the
#' data: Gleditsch, Kristian S. & Michael D. Ward. 1999. "Interstate System
#' Membership: A Revised List of the Independent States since 1816."
#' International Interactions 25: 393-413} \item{year}{year of observation}
#' \item{pop}{population, see also notes below} \item{realgdp}{total real GDP,
#' 2005} \item{rgdppc}{real GDP per capita, 2005 prices} \item{cgdppc}{GDP per
#' capita, current prices} \item{origin}{origin code for observation} }
#'
#' @details
#'
#' These data are explained in greater detail in Gleditsch, Kristian S. 2002.
#' "Expanded Trade and GDP Data," Journal of Conflict Resolution 46: 712-24.
#' \url{http://jcr.sagepub.com/content/46/5/712.short}.
#'
#' \emph{Table of Origin Codes} \itemize{
#' \item 0 - From PWT 8.0 (but see
#' explanations for population figures below)
#' \item -1 - From PWT 5.6
#' \item -2 - From Maddison Project Database
#' \item 1 - Imputations for lead/tails based on first/last available value,
#' deflated to current value for gdppc using the GDP deflator from the Bureau
#' of Economic Analysis
#' \item 2 Interpolated value
#' (within series)
#' \item 3 Estimates based on figures from World Bank Global
#' Development Indicators, using shares to reference countries. (See
#' ksgmdw/scaling.asc for details)}
#'
#' \emph{Population figures: Origins and exceptions}
#'
#' a) All missing population figures in the PWT data have been replaced by
#' population figures from the Expanded Population Data v.2.0, compiled by
#' Kristian Skrede Gleditsch. This file is is provided as
#' ksgmdw/countrypopestimates.asc, see the associated readme file for details
#' on the origin codes. No documention is available as of yet for these data.
#' These estimates are generally based on country specific censuses, and the
#' particular references for each are available on request.
#'
#' b) The PWT populations estimates have been retained when available, with
#' some exceptions
#'
#' The PWT project lists population figures for countries that experience
#' large boundary changes based on the current size of the existing state.
#' This means that the PWT population figures for the Federal Republic of
#' Germany *before* the end of the German Democratic Republic reflect the
#' population size of the two Germanies combined. Likewise, pre-1991
#' population figures for the Soviet Union reflect the size of present day
#' Russia, even though this is only about half the size of former Soviet
#' Union. I am grateful to Ye Wang from the PWT project for explicity
#' confirming this to me.
#'
#' Since the present size of a state seems inappropriate for historical
#' comparisons, the current version of the data replaces the PWT figures with
#' estimates from the Expanded Population Data v.2.0 in the following
#' instances:
#'
#' A. Population of states prior to merging with other states i) Federal
#' Republic of Germany through 1990 (prior to inclusion of former Democratic
#' Republic of Germany) ii) Democratic Republic of Vietnam through 1975 (prior
#' to inclusion of former Republic of (South) Vietnam iii) Arab Republic of
#' (North) Yemen through 1990 (prior to merger with former People's Republic
#' of (South) Yemen
#'
#' B. Population of states prior to secession of new states i) Pakistan
#' through 1970 (prior to secession of Bangladesh) ii) Soviet Union through
#' 1990 (prior to the secession of multiple former Soviet Republics and the
#' new Russian Federation) iii) Yugoslavia through 1991 (prior to the
#' independence of Bosnia, Croatia, Slovenia, and Macedonia)
#'
#' @source \url{http://privatewww.essex.ac.uk/~ksg/exptradegdp.html}
"expanded_gdp"
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