present.R

#' Male and female births in the US
#'
#' Counts of the total number of male and female births in the United States 
#' from 1940 to 2013.
#'
#' @format A tbl_df with 74 rows and 3 variables:
#' \describe{
#'   \item{year}{year, ranging from 1940 to 2013}
#'   \item{boys}{number of male births}
#'   \item{girls}{number of female births}
#' }
#' @source Data up to 2002 appear in Mathews TJ, and Hamilton BE. 2005. Trend
#' analysis of the sex ratio at birth in the United States. National Vital
#' Statistics Reports 53(20):1-17. Data for 2003 - 2013 have been collected
#' from annual National Vital Statistics Reports published by the US Department 
#' of Health and Human Services, Centers for Disease Control and Prevention, 
#' National Center for Health Statistics.
"present"