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#' Predict hematocrit in infants under 1 year old.
#'
#' For infants under 1 year, hematocrit was not measured in NHANES. Assume a
#' log-normal distribution where plus/minus 1 standard deviation of the
#' underlying normal distribution is given by the reference range. Draw
#' hematocrit values from these distributions by age.
#'
#' \tabular{cc}{
#' Age \tab Reference range\cr
#' <1 month \tab 31-49\cr
#' 1-6 months \tab 29-42\cr
#' 7-12 months \tab 33-38
#' }
#'
#' @param age_months Vector of ages in months; all must be <= 12.
#'
#' @return Vector of hematocrit percentages corresponding to the input vector
#' of ages.
#'
#'@keywords httk-pop
#'
#'@author Caroline Ring
#'
#'@references
#' \insertRef{ring2017identifying}{httk}
#' @import stats
hematocrit_infants <- function(age_months){
#For infants under 1 year,
#hematocrit was not measured in NHANES.
#Assume a log-normal distribution where plus/minus 1 standard deviation
#of the underlying normal distribution is given by the reference range
#The 1-sd range is based on the fact that for 1year olds,
#the NHANES data shows approximately plus and minus 1 sd
#within 33-38% hematocrit, which is the reference range for that age.
loghct <- rep(NA, length(age_months))
if (length(loghct[age_months<1])>0){
#For infants under 1 month old
#reference range 31-49
sig.tmp <- (log(49)-log(31))/2
mu.tmp <- log(49) - sig.tmp
loghct[age_months<1] <- rnorm(n=length(loghct[age_months<1]),
mean=mu.tmp,
sd=sig.tmp)
}
if (length(loghct[age_months>=1 &
age_months<=6])>0){
#For infants 1-6 months old
sig.tmp <- (log(42)-log(29))/2
mu.tmp <- log(42) - sig.tmp
loghct[age_months>=1 &
age_months<=6] <- rnorm(n=length(loghct[age_months>=1 &
age_months<=6]),
mean=mu.tmp,
sd=sig.tmp)
}
if (length(loghct[age_months>6 &
age_months<12])>0){
#For infants 7-12 months old
sig.tmp <- (log(38)-log(33))/2
mu.tmp <- log(38) - sig.tmp
loghct[age_months>6 &
age_months<12] <- rnorm(n=length(loghct[age_months>6 &
age_months<12]),
mean=mu.tmp,
sd=sig.tmp)
}
hct <- exp(loghct)
return(hct)
}
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