X-Git-Url: https://git.nmode.ca/Rnaught/blobdiff_plain/9c1a5668803e735f034700c55028ffc0146f1e93..a50ca5855eecf12908327252d627df3af076fc88:/R/seqB.R?ds=inline diff --git a/R/seqB.R b/R/seqB.R index 0f8a1b9..8685f39 100644 --- a/R/seqB.R +++ b/R/seqB.R @@ -7,43 +7,42 @@ #' The distribution of R0 is then updated sequentially, with one update for each new case count observation. #' The final estimate of R0 is \code{Rhat}, the mean of the (last) posterior distribution. #' The prior distribution is the initial belief of the distribution of R0; which in this implementation is the uninformative uniform -#' distribution with values between zero and \code{kappa}. Users can change the value of kappa only (ie. the prior distribution +#' distribution with values between zero and \code{kappa}. Users can change the value of /code{kappa} only (i.e., the prior distribution #' cannot be changed from the uniform). As more case counts are observed, the influence of the prior distribution should lessen on #' the final estimate \code{Rhat}. #' #' This method is based on an approximation of the SIR model, which is most valid at the beginning of an epidemic. The method assumes #' that the mean of the serial distribution (sometimes called the serial interval) is known. The final estimate can be quite sensitive -#' to this value, so sensitivity testing is strongly recommended. Users should be careful about units of time (e.g. are counts observed +#' to this value, so sensitivity testing is strongly recommended. Users should be careful about units of time (e.g., are counts observed #' daily or weekly?) when implementing. #' #' Our code has been modified to provide an estimate even if case counts equal to zero are present in some time intervals. This is done #' by grouping the counts over such periods of time. Without grouping, and in the presence of zero counts, no estimate can be provided. #' -#' @param NT Vector of case counts -#' @param mu Mean of the serial distribution (needs to match case counts in time units; for example, if case counts are -#' weekly and the serial distribution has a mean of seven days, then \code{mu} should be set to one, if case -#' counts are daily and the serial distribution has a mean of seven days, then \code{mu} should be set to seven) -#' @param kappa Largest possible value of uniform prior, defaults to 20. This describes the prior belief on ranges of R0, -#' so should be set to a higher value if R0 is believed to be larger. +#' @param NT Vector of case counts. +#' @param mu Mean of the serial distribution. This needs to match case counts in time units. For example, if case counts +#' are weekly and the serial distribution has a mean of seven days, then \code{mu} should be set to one. If case +#' counts are daily and the serial distribution has a mean of seven days, then \code{mu} should be set to seven. +#' @param kappa Largest possible value of uniform prior (defaults to 20). This describes the prior belief on ranges of R0, +#' and should be set to a higher value if R0 is believed to be larger. #' -#' @return secB returns a list containing the following components: \code{Rhat} is the estimate of R0 (the posterior mean), -#' \code{posterior} is the posterior distribution of R0 from which alternate estimates can be obtained (see examples), -#' \code{group} is an indicator variable (if \code{group=TRUE}, zero values of NT were input and grouping was done to -#' obtain \code{Rhat}), and \code{inputs} is a list of the original input variables \code{NT, gamma, kappa}. The variable -#' \code{posterior} is returned as a list made up of \code{supp} the support of the distribution and \code{pmf} the -#' probability mass function. +#' @return \code{secB} returns a list containing the following components: \code{Rhat} is the estimate of R0 (the posterior mean), +#' \code{posterior} is the posterior distribution of R0 from which alternate estimates can be obtained (see examples), +#' and \code{group} is an indicator variable (if \code{group=TRUE}, zero values of NT were input and grouping was done +#' to obtain \code{Rhat}). The variable \code{posterior} is returned as a list made up of \code{supp} (the support of +#' the distribution) and \code{pmf} (the probability mass function). #' #' @examples #' ## ===================================================== ## #' ## Illustrate on weekly data ## #' ## ===================================================== ## #' -#' NT <- c(1, 4, 10, 5, 3, 4, 19, 3, 3, 14, 4) +#' NT <- c(1, 4, 10, 5, 3, 4, 19, 3, 3, 14, 4) #' ## obtain Rhat when serial distribution has mean of five days -#' res1 <- seqB(NT=NT, mu=5/7) +#' res1 <- seqB(NT=NT, mu=5/7) #' res1$Rhat #' ## obtain Rhat when serial distribution has mean of three days -#' res2 <- seqB(NT=NT, mu=3/7) +#' res2 <- seqB(NT=NT, mu=3/7) #' res2$Rhat #' #' ## ============================================================= ## @@ -51,26 +50,27 @@ #' ## ============================================================= ## #' #' Rpost <- res1$posterior -#' loc <- which(Rpost$pmf==max(Rpost$pmf)) -#' Rpost$supp[loc] # posterior mode -#' res1$Rhat # compare with posterior mean +#' loc <- which(Rpost$pmf == max(Rpost$pmf)) +#' Rpost$supp[loc] # posterior mode +#' res1$Rhat # compare with posterior mean #' -#' par(mfrow=c(2,1), mar=c(2,2,1,1)) +#' par(mfrow=c(2, 1), mar=c(2, 2, 1, 1)) #' plot(Rpost$supp, Rpost$pmf, col="black", type="l", xlab="", ylab="") #' abline(h=1/(20/0.01+1), col="red") #' abline(v=res1$Rhat, col="blue") #' abline(v=Rpost$supp[loc], col="purple") -#' legend("topright", legend=c("prior", "posterior", "posterior mean (Rhat)", "posterior mode"), col=c("red", "black", "blue", "purple"), lty=1) -#' plot(Rpost$supp, Rpost$pmf, col="black", type="l", xlim=c(0.5,1.5), xlab="", ylab="") +#' legend("topright", legend=c("prior", "posterior", "posterior mean (Rhat)", "posterior mode"), +#' col=c("red", "black", "blue", "purple"), lty=1) +#' plot(Rpost$supp, Rpost$pmf, col="black", type="l", xlim=c(0.5, 1.5), xlab="", ylab="") #' abline(h=1/(20/0.01+1), col="red") #' abline(v=res1$Rhat, col="blue") #' abline(v=Rpost$supp[loc], col="purple") -#' legend("topright", legend=c("prior", "posterior", "posterior mean (Rhat)", "posterior mode"), col=c("red", "black", "blue", "purple"), lty=1) +#' legend("topright", legend=c("prior", "posterior", "posterior mean (Rhat)", "posterior mode"), +#' col=c("red", "black", "blue", "purple"), lty=1) #' #' ## ========================================================= ## #' ## Compute Rhat using only the first five weeks of data ## #' ## ========================================================= ## -#' #' #' res3 <- seqB(NT=NT[1:5], mu=5/7) # serial distribution has mean of five days #' res3$Rhat @@ -121,6 +121,6 @@ seqB <- function(NT, mu, kappa=20) { Rhat <- sum(R * posterior) - return(list(Rhat=Rhat, posterior=list(supp=R, pmf=posterior), group=group, inputs=list(NT=NT, mu=mu, kappa=kappa))) + return(list(Rhat=Rhat, posterior=list(supp=R, pmf=posterior), group=group)) } }