diff --git a/NAMESPACE b/NAMESPACE
index d75f824..1015c5a 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -1 +1,2 @@
 exportPattern("^[[:alpha:]]+")
+importFrom("stats", "quantile")
\ No newline at end of file
diff --git a/R/meanimpute.R b/R/meanimpute.R
index cc7cf5e..c3fe624 100644
--- a/R/meanimpute.R
+++ b/R/meanimpute.R
@@ -1,4 +1,6 @@
 #' Meanimputation
+#' @param x A vector.
+#'
 #' @export
 meanimpute <- function(x) {
   x[is.na(x)] <- mean(x, na.rm = TRUE)
diff --git a/R/transform_log.R b/R/transform_log.R
new file mode 100644
index 0000000..157bd49
--- /dev/null
+++ b/R/transform_log.R
@@ -0,0 +1,16 @@
+#' Transform_log
+#' 
+#' Log transformation.
+#' 
+#' @param x A vector.
+#' 
+#' @examples
+#' transform_log(exp(rnorm(2)))
+#' 
+#' @export
+
+transform_log <- function(x) {
+  if (any(x<0)) {stop("input can't be negative")}
+  x <- log(x)
+  x
+}
\ No newline at end of file
diff --git a/R/windsorize.R b/R/windsorize.R
index b4e15e6..9438ac7 100644
--- a/R/windsorize.R
+++ b/R/windsorize.R
@@ -1,10 +1,22 @@
 #' Windsorize
-#'
-#' Do some windsorization.
+#' 
+#' Winsorizing or winsorization is the transformation of statistics by limiting
+#' extreme values in the statistical data to reduce the effect of possibly
+#' spurious outliers.
+#' 
+#' @param x A vector.
+#' @param p A quantile.
+#' 
+#' @examples
+#' windsorize(rnorm(5))
+#' 
 #' @export
 windsorize <- function(x, p = .90) {
-  q <- quantile(x, p)
-  x[x >= q] <- q
+  if(all(is.na(x))) {stop("argument should not be a vector containing only NA-s or NULL-s")}
+  q_up <- quantile(x, 0.5 + p / 2 )
+  q_down <- quantile(x, 0.5 - p / 2 )
+  x[x >= q_up] <- q_up
+  x[x <= q_down] <- q_down
   x
 }
 
diff --git a/man/meanimpute.Rd b/man/meanimpute.Rd
index 8139e8f..7773d13 100644
--- a/man/meanimpute.Rd
+++ b/man/meanimpute.Rd
@@ -6,6 +6,9 @@
 \usage{
 meanimpute(x)
 }
+\arguments{
+\item{x}{A vector.}
+}
 \description{
 Meanimputation
 }
diff --git a/man/transform_log.Rd b/man/transform_log.Rd
new file mode 100644
index 0000000..e77f2c3
--- /dev/null
+++ b/man/transform_log.Rd
@@ -0,0 +1,18 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/transform_log.R
+\name{transform_log}
+\alias{transform_log}
+\title{Transform_log}
+\usage{
+transform_log(x)
+}
+\arguments{
+\item{x}{A vector.}
+}
+\description{
+Log transformation.
+}
+\examples{
+transform_log(exp(rnorm(2)))
+
+}
diff --git a/man/windsorize.Rd b/man/windsorize.Rd
index 832c3cb..249fecc 100644
--- a/man/windsorize.Rd
+++ b/man/windsorize.Rd
@@ -6,6 +6,17 @@
 \usage{
 windsorize(x, p = 0.9)
 }
+\arguments{
+\item{x}{A vector.}
+
+\item{p}{A quantile.}
+}
 \description{
-Do some windsorization.
+Winsorizing or winsorization is the transformation of statistics by limiting
+extreme values in the statistical data to reduce the effect of possibly
+spurious outliers.
+}
+\examples{
+windsorize(rnorm(5))
+
 }
diff --git a/tests/testthat.R b/tests/testthat.R
new file mode 100644
index 0000000..8adbfff
--- /dev/null
+++ b/tests/testthat.R
@@ -0,0 +1,4 @@
+library(testthat)
+library(datacleaner)
+
+test_check("datacleaner")
diff --git a/tests/testthat/test_transform_log.R b/tests/testthat/test_transform_log.R
new file mode 100644
index 0000000..9b96c99
--- /dev/null
+++ b/tests/testthat/test_transform_log.R
@@ -0,0 +1,5 @@
+context("No negative values")
+library(datacleaner)
+test_that("There are no negative values in input", {
+  expect_error(transform_log(c(1,2,-1)), "input can't be negative")
+})
\ No newline at end of file
diff --git a/tests/testthat/test_windsorize.R b/tests/testthat/test_windsorize.R
new file mode 100644
index 0000000..4e19ba5
--- /dev/null
+++ b/tests/testthat/test_windsorize.R
@@ -0,0 +1,6 @@
+context("NA and null values")
+library(datacleaner)
+test_that("NA and null values produce error message", {
+  expect_error(windsorize(NA), "argument should not be a vector containing only NA-s or NULL-s")
+  expect_error(windsorize(NULL), "argument should not be a vector containing only NA-s or NULL-s")
+})
\ No newline at end of file