diff --git a/ecsades/DESCRIPTION b/ecsades/DESCRIPTION
index be6827d..84d5a5c 100644
--- a/ecsades/DESCRIPTION
+++ b/ecsades/DESCRIPTION
@@ -1,8 +1,8 @@
 Package: ecsades
 Type: Package
 Title: Environmental contours for safe design of ships and other marine structures 
-Version: 1.0.0
-Date: 2018-12-19
+Version: 1.0.1
+Date: 2019-01-24
 Author: Ye Liu
 Maintainer: Ye Liu <y.liu@hrwallingford.com>
 Description: 
diff --git a/ecsades/R/estimate_iform.r b/ecsades/R/estimate_iform.r
index 2e82fef..5afaf67 100644
--- a/ecsades/R/estimate_iform.r
+++ b/ecsades/R/estimate_iform.r
@@ -141,8 +141,8 @@ estimate_iform = function(
   calc[, p1:=pnorm(u1)]
   calc[, p2:=pnorm(u2)]
   calc[, hs:=qweibull(p1, shape=wln$hs$par["shape"], scale=wln$hs$par["scale"])+wln$hs$par["loc"]]
-  calc[, m:=wln$tp$par["d"] + wln$tp$par["e"] * log(hs + wln$tp$par["f"])]
-  calc[, s:=sqrt(wln$tp$par["g"] + wln$tp$par["k"] * exp(wln$tp$par["m"] * (hs ^ wln$tp$par["q"])))]
+  calc[, m:=wln$tp$par[1] + wln$tp$par[2] * (hs ^ wln$tp$par[3])]
+  calc[, s:=wln$tp$par[4] + wln$tp$par[5] * exp(wln$tp$par[6] * hs)]
   calc[, tp:=qlnorm(p2, m, s)]
   
   # Return
diff --git a/ecsades/R/fit_wln.r b/ecsades/R/fit_wln.r
index f1d7197..3483a24 100644
--- a/ecsades/R/fit_wln.r
+++ b/ecsades/R/fit_wln.r
@@ -4,8 +4,7 @@
 #' This function fits a Weibull-log-normal (\code{ht}) model to the given wave data, such that the wave height
 #' \code{hs} follows a translated (or 3-parameter) Weibull distribution, and the wave period given the wave
 #' follows a conditional log-normal distributuion with the location and scale parameters as functions
-#' of the corresponding \code{hs} value. The formulation of the conditional log-normal distribution follows that
-#' proposed in Haver and Winterstein (2008).
+#' of the corresponding \code{hs} value.
 #' 
 #' @param data the wave data in the form a \code{data.table} with wave height \code{hs} and wave period
 #' \code{tp} as columns.
@@ -13,10 +12,33 @@
 #' @param npy the number of data points per year, usually estimated by the number of rows in the
 #' supplied wave data divided by the total period of data coverage (in years).
 #' 
+#' @param hs_constraint_range a multiplier to the maximum \code{hs} in the data within such that the standard
+#' deviation of the log-normal conditional distribution of \code{tp} given \code{hs} is constrained to be strictly
+#' positive (see details for more information) over this user-defined range. The default value is 1.5.
+#' 
+#' @param weighted_tp_fit whether or not the estimation of the conditional model for \code{tp} uses a weighted
+#' likelihood (see details). The default option is \code{FALSE}.
+#' 
 #' @details
 #' The input \code{data} must be a \code{data.table} object with \code{hs} and \code{tp} columns. This can be
 #' generated by reading a CSV file using function \code{\link[data.table]{fread}}.
 #' 
+#' The formulation of the conditional distribution is given by
+#' \deqn{log(tp | hs=h) ~ N(\mu(h), \sigma(h)^2)}
+#' where the mean and the standard deviation are
+#' \deqn{\mu(h) = a_0 + a_1 h^a_2 and \sigma(h) = b_0 + b_1 exp(h * b_2)}
+#' 
+#' The strictly positive constraint for the standard deviation has a large influence on the estimated values for
+#' coefficients \eqn{b_0}, \eqn{b_1} and \eqn{b_2}. In general, enforcing the standardard deviation to be strictly
+#' positive over a larger range (larger \code{hs_constraint_range}) may result in a poorer fit of the model
+#' to the provided observation data.
+#' 
+#' There is an option to use weighted likelihood when fitting the log-normal distribution to \code{tp|hs}. When this
+#' option is used, the observation data are divided to bins of equal sizes (every whole number in \code{hs}) and the
+#' weight for each data point is inversely proportional to the total number of points within the same bin.  This
+#' option effectively weighs up points in the tail of the distribution, but weighs down the points in the centre
+#' of the distribution.
+#' 
 #' @return An joint distribution object of class \code{wln} containing the key information of a fitted
 #' Weibull-log-normal model, including the three parameters of the Weibull distribution for \code{hs}
 #' and the seven parameters of the conditional log-normal distribution for \code{tp} given \code{tp}
@@ -28,6 +50,9 @@
 #' # Fit Weibull-log-normal distribution 
 #' noaa_wln = fit_wln(data = noaa_ww3, npy = nrow(noaa_ww3)/10)
 #' 
+#' # Fit Weibull-log-normal distribution with additional options
+#' noaa_wln2 = fit_wln(data = noaa_ww3, npy = nrow(noaa_ww3)/10, hs_constraint_range = 3, weighted_tp_fit = TRUE)
+#' 
 #' @references 
 #' Haver, Sverre & Winterstein, Steven. (2009). Environmental Contour Lines: A Method for Estimating Long
 #' Term Extremes by a Short Term Analysis. Transactions - Society of Naval Architects and Marine Engineers. 116. 
@@ -35,28 +60,32 @@
 #' @seealso \code{\link{fit_ht}}, \code{\link{sample_jdistr}}
 #' 
 #' @export
-fit_wln = function(data, npy){
+fit_wln = function(data, npy, hs_constraint_range = 1.5, weighted_tp_fit = FALSE){
   
   res = list()
   class(res) = "wln"
   res$npy = npy
   res$hs = .fit_weibull(data = data$hs)
-  res$tp = .fit_iform_lnorm(hs = data$hs, tp = data$tp)
+  res$tp = .fit_iform_lnorm(
+    hs = data$hs, tp = data$tp,
+    hs_constraint_range = hs_constraint_range, weighted_tp_fit = weighted_tp_fit)
   return(res)
 }
 
 
 # Conditional log-normal used for IFROM -----------------------------------
 
-.fit_iform_lnorm = function(hs, tp){
+.fit_iform_lnorm = function(hs, tp, hs_constraint_range, weighted_tp_fit){
   
   log_tp = log(tp)
-  # theta = c(d, e, f, g, k, m, q)
-  theta0 = c(d=mean(log_tp), e=0, f=0, g=sd(log_tp), k=0, m=0, q=0)
+  theta0 = c(mean(log_tp), 0, 0, sd(log_tp), 0, 0)
   op = optim(
     par = theta0,
     fn = .nll_iform_lnorm,
-    log_tp = log_tp, hs = hs, control = list(maxit=2e3))
+    log_tp = log_tp, hs = hs,
+    hs_constraint_range = hs_constraint_range,
+    weighted_tp_fit = weighted_tp_fit,
+    control = list(maxit=2e3))
   
   res = list()
   class(res) = "iform_lnorm"
@@ -66,22 +95,34 @@ fit_wln = function(data, npy){
   return(res)
 }
 
-.nll_iform_lnorm = function(theta, log_tp, hs){
+.nll_iform_lnorm = function(theta, log_tp, hs, hs_constraint_range, weighted_tp_fit){
   
   # Constraints
-  if(theta[3] <= 0 | theta[6]>0 | theta[4]<0 | theta[4]+theta[5]<0){
+  mean_range = theta[1] + theta[2] * c(max(hs)*hs_constraint_range, .limit_zero)^theta[3]
+  sd_range = theta[4] + theta[5] * exp(theta[6] * c(max(hs)*hs_constraint_range, .limit_zero))
+  
+  if(any(is.na(sd_range)) || any(is.na(mean_range)) || min(sd_range)<.limit_zero){
     return(.limit_inf)
   }
   
   # mean & sd
-  norm_mean = theta[1] + theta[2] * log(hs + theta[3])
-  norm_var = theta[4] + theta[5] * exp(theta[6] * (hs ^ theta[7]))
-  if (any(norm_var <= 0)){
+  norm_mean = theta[1] + theta[2]* (hs^theta[3])
+  norm_sd = theta[4] + theta[5] * exp(theta[6] * hs)
+  if (any(norm_sd <= 0)){
     return(.limit_inf)
   }
   
+  # Weights
+  if(weighted_tp_fit){
+    freq = data.table(hs, rhs =round(hs*2))
+    freq[, weight:=1/.N, .(rhs)]
+    weight = freq$weight
+  }else{
+    weight = 1
+  }
+  
   # return
-  nll = -dnorm(log_tp, norm_mean, sqrt(norm_var), log=TRUE)
+  nll = -dnorm(log_tp, norm_mean, norm_sd, log=TRUE)*weight - log_tp*weight
   res = min(.limit_inf, sum(nll))
   return(res)
 }
diff --git a/ecsades/R/sample_jdistr.r b/ecsades/R/sample_jdistr.r
index b583e60..0a4cdfd 100644
--- a/ecsades/R/sample_jdistr.r
+++ b/ecsades/R/sample_jdistr.r
@@ -110,9 +110,9 @@ sample_jdistr = function(jdistr, sim_year){
   hs = rweibull(n_sim, shape = wln$hs$par["shape"], scale = wln$hs$par["scale"]) + wln$hs$par["loc"]
   
   # sample tp cond on hs
-  tp_norm_mean = wln$tp$par[1] + wln$tp$par[2] * log(hs + wln$tp$par[3])
-  tp_norm_var = wln$tp$par[4] + wln$tp$par[5] * exp(wln$tp$par[6] * (hs ^ wln$tp$par[7]))
-  tp = exp(rnorm(n_sim, tp_norm_mean, sqrt(tp_norm_var)))
+  tp_norm_mean = wln$tp$par[1] + wln$tp$par[2] * (hs ^ wln$tp$par[3])
+  tp_norm_sd = wln$tp$par[4] + wln$tp$par[5] * exp(wln$tp$par[6] * hs)
+  tp = exp(rnorm(n_sim, tp_norm_mean, tp_norm_sd))
   
   res = data.table(hs, tp)
   return(res)
diff --git a/ecsades/man/ecsades-package.Rd b/ecsades/man/ecsades-package.Rd
index 8c192e1..a6e8985 100644
--- a/ecsades/man/ecsades-package.Rd
+++ b/ecsades/man/ecsades-package.Rd
@@ -1,5 +1,5 @@
 % Generated by roxygen2: do not edit by hand
-% Please edit documentation in R/ecsades.R
+% Please edit documentation in R/ecsades.r
 \docType{package}
 \name{ecsades-package}
 \alias{ecsades}
diff --git a/ecsades/man/fit_wln.Rd b/ecsades/man/fit_wln.Rd
index 29bef7c..50b2933 100644
--- a/ecsades/man/fit_wln.Rd
+++ b/ecsades/man/fit_wln.Rd
@@ -4,7 +4,7 @@
 \alias{fit_wln}
 \title{Fitting Weibull-log-normal model to wave data}
 \usage{
-fit_wln(data, npy)
+fit_wln(data, npy, hs_constraint_range = 1.5, weighted_tp_fit = FALSE)
 }
 \arguments{
 \item{data}{the wave data in the form a \code{data.table} with wave height \code{hs} and wave period
@@ -12,6 +12,13 @@ fit_wln(data, npy)
 
 \item{npy}{the number of data points per year, usually estimated by the number of rows in the
 supplied wave data divided by the total period of data coverage (in years).}
+
+\item{hs_constraint_range}{a multiplier to the maximum \code{hs} in the data within such that the standard
+deviation of the log-normal conditional distribution of \code{tp} given \code{hs} is constrained to be strictly
+positive (see details for more information) over this user-defined range. The default value is 1.5.}
+
+\item{weighted_tp_fit}{whether or not the estimation of the conditional model for \code{tp} uses a weighted
+likelihood (see details). The default option is \code{FALSE}.}
 }
 \value{
 An joint distribution object of class \code{wln} containing the key information of a fitted
@@ -22,12 +29,27 @@ and the seven parameters of the conditional log-normal distribution for \code{tp
 This function fits a Weibull-log-normal (\code{ht}) model to the given wave data, such that the wave height
 \code{hs} follows a translated (or 3-parameter) Weibull distribution, and the wave period given the wave
 follows a conditional log-normal distributuion with the location and scale parameters as functions
-of the corresponding \code{hs} value. The formulation of the conditional log-normal distribution follows that
-proposed in Haver and Winterstein (2008).
+of the corresponding \code{hs} value.
 }
 \details{
 The input \code{data} must be a \code{data.table} object with \code{hs} and \code{tp} columns. This can be
 generated by reading a CSV file using function \code{\link[data.table]{fread}}.
+
+The formulation of the conditional distribution is given by
+\deqn{log(tp | hs=h) ~ N(\mu(h), \sigma(h)^2)}
+where the mean and the standard deviation are
+\deqn{\mu(h) = a_0 + a_1 h^a_2 and \sigma(h) = b_0 + b_1 exp(h * b_2)}
+
+The strictly positive constraint for the standard deviation has a large influence on the estimated values for
+coefficients \eqn{b_0}, \eqn{b_1} and \eqn{b_2}. In general, enforcing the standardard deviation to be strictly
+positive over a larger range (larger \code{hs_constraint_range}) may result in a poorer fit of the model
+to the provided observation data.
+
+There is an option to use weighted likelihood when fitting the log-normal distribution to \code{tp|hs}. When this
+option is used, the observation data are divided to bins of equal sizes (every whole number in \code{hs}) and the
+weight for each data point is inversely proportional to the total number of points within the same bin.  This
+option effectively weighs up points in the tail of the distribution, but weighs down the points in the centre
+of the distribution.
 }
 \examples{
 # Load data
@@ -36,6 +58,9 @@ data(noaa_ww3)
 # Fit Weibull-log-normal distribution 
 noaa_wln = fit_wln(data = noaa_ww3, npy = nrow(noaa_ww3)/10)
 
+# Fit Weibull-log-normal distribution with additional options
+noaa_wln2 = fit_wln(data = noaa_ww3, npy = nrow(noaa_ww3)/10, hs_constraint_range = 3, weighted_tp_fit = TRUE)
+
 }
 \references{
 Haver, Sverre & Winterstein, Steven. (2009). Environmental Contour Lines: A Method for Estimating Long