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#' Sample size for estimating single mean
#' @description This function computes adequate sample size based on the method of
#' estimating mean with absolute or relative precision. It can be used for descriptive studies
#' where the researcher wishes to describe the distribution of one or more quantitative outcome variables
#' without looking at their causal relationship and hypothesis testing.
#'
#' @param mean anticipated population mean (required if relative precision is desired otherwise not required)
#' @param sig anticipated population standard deviation
#' @param prec desired level of precision on either side of the population mean
#' @param alp level of significance or accepted level of probability of type I error
#' @param relative a logical argument indicating relative or absolute precision (\code{FALSE} gives absolute precision)
#' @import stringi stats
#' @export
#' @return number needed to estimate mean within the desired precision level
#' @author R. Amala, Scientist-C, ICMR-VCRC, Puducherry & G. Kumarapandiyan, Asst. Prof., Madras Christian College, Chennai
#' @references
#' Lwanga, S. K., Lemeshow, S., & World Health Organization. (1991). Sample size determination in health studies: a practical manual. World Health Organization.
#' @import stringi
#' @examples
#' estm(sig=6.3,prec=1.2,alp=0.05,relative=FALSE)
#' estm(mean=14,sig=8,prec=0.1,alp=0.05,relative = TRUE)
estm<-function(mean,sig,prec,alp,relative=FALSE) {
if(relative==FALSE)
{
n<-ceiling(( (qnorm(1-alp/2))^2*sig^2)/prec^2)
if (!requireNamespace("stringi", quietly = TRUE)) {
stop("Package 'stringi' is not installed.")
}
s <- stringi::stri_paste("Description: \n Sample size is estimated with an expected standard deviation of the 'outcome of interest' among the 'population' as ", sig, " units at ", alp*100, "% level of significance with an absolute precision of ",prec, " units, the estimated sample size is ",n, ".\n")
out <- list(Sample_Size = n, stringi::stri_pad(stringi::stri_wrap(s), side='both'))
return(out)
}
else
{
n<-ceiling(( (qnorm(1-alp/2))^2*sig^2)/(mean*prec)^2)
if (!requireNamespace("stringi", quietly = TRUE)) {
stop("Package 'stringi' is not installed.")
}
s <- stri_paste("Description: Sample size is estimated with an expected standard deviation of the 'outcome of interest' among the 'population' as ", sig, " units at ", alp*100, "% level of significance with a Relative precision of ",prec, " units, the estimated sample size is ", n , ".\n")
out <- list(Sample_Size = n, stringi::stri_pad(stringi::stri_wrap(s), side='both'))
return(out)
}
}
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