 # How can we create right/left skewed normal distribution curve in R ?

How can we create skewed normal distribution curve in R ?
How to vary the skewedness using a variable ?
i.e. height of the peak and tail of the plot ?

Although many links say that they have an answer but non worked

This is in addition to earlier question:
How to calculate the cumulative area of the overlapped normal distribution curve in R ? solved by @Yarnabrina

``````# Tried
library(fGarch)
library(tidyverse)
N <- 10000
x <- rnbinom(N, 10, .5)
dsnorm(x, mean = 0, sd = 1, xi = 1.5, log = FALSE)
# psnorm(x, mean = 0, sd = 1, xi = 1.5) %>% plot()
# qsnorm(x, mean = 0, sd = 1, xi = 1.5) %>% plot()
# rsnorm(x, mean = 0, sd = 1, xi = 1.5) %>% plot()

plot(dsnorm(x, mean = 0, sd = 1, xi = 1.5, log = FALSE))``````
1 Like

Your links suggest that you have used the sn package (https://cran.r-project.org/web/packages/sn/index.html) though the code you post does not mention it. In what way did sn not work?
Quoting from the sn package documentation -
*"dsn: Skew-Normal Distribution *
Description: Density function, distribution function, quantiles and random number generation for the skew-normal (SN) and the extended skew-normal (ESN) distribution."

I'm not sure I understand your problem. What exactly did you expect your result will be with this code? Note that `x` is not sorted, and you're plotting the points against `1:10000`.

Here's an way to plot with `ggplot2`. I simulated in the range `[-5, 5]`, but you can change as you prefer.

Note that the implementations in the packages `fGarch` and `sn` vary.

``````library(fGarch)
library(ggplot2)
library(sn)
#>
#> Attaching package: 'sn'
#> The following object is masked from 'package:fBasics':
#>
#>     vech
#> The following object is masked from 'package:stats':
#>
#>     sd

central_tendency_sim <- 0
dispersion_sim <- 1
skewness_sim <- 1.5

N_sim <- 10000

obs_sim <- seq(from = -5,
to = 5,
length.out = N_sim)

ggplot(data = data.frame(u = obs_sim),
mapping = aes(x = u)) +
stat_function(mapping = aes(colour = "fGarch"),
fun = dsnorm,
args = list(mean = central_tendency_sim,
sd = dispersion_sim,
xi = skewness_sim)) +
stat_function(mapping = aes(colour = "sn"),
fun = dsn,
args = list(xi = central_tendency_sim,
omega = dispersion_sim,
alpha = skewness_sim)) +
stat_function(mapping = aes(colour = "standard"),
fun = dnorm) +
scale_colour_manual(name = NULL,
values = c("red", "blue", "green"))
`````` Created on 2019-09-06 by the reprex package (v0.3.0)

In case you want to plot in base R, that also works.
``````library(fGarch)
library(sn)
#>
#> Attaching package: 'sn'
#> The following object is masked from 'package:fBasics':
#>
#>     vech
#> The following object is masked from 'package:stats':
#>
#>     sd

central_tendency_sim <- 0
dispersion_sim <- 1
skewness_sim <- 1.5

N_sim <- 10000

obs_sim <- seq(from = -5,
to = 5,
length.out = N_sim)

den_sim_fGarch <- dsnorm(x = obs_sim,
mean = central_tendency_sim,
sd = dispersion_sim,
xi = skewness_sim)

den_sim_sn <- dsn(x = obs_sim,
xi = central_tendency_sim,
omega = dispersion_sim,
alpha = skewness_sim)

par(mfrow = c(2, 2))

plot(x = obs_sim,
y = den_sim_fGarch,
type = "l",
xlab = "observations",
ylab = "densities",
main = "plot points and join - fGarch",
col = "violetred")
curve(expr = dnorm,
from = -5,
to = 5,
n = N_sim,
col = "limegreen",
lty = 2)

plot(x = obs_sim,
y = den_sim_sn,
type = "l",
xlab = "observations",
ylab = "densities",
main = "plot points and join - sn",
col = "navyblue")
curve(expr = dnorm,
from = -5,
to = 5,
n = N_sim,
col = "limegreen",
lty = 2)

plot.function(x = function(t) dsnorm(x = t,
mean = central_tendency_sim,
sd = dispersion_sim,
xi = skewness_sim),
to = 5,
from = -5,
n = N_sim,
xlab = "observations",
ylab = "densities",
main = "plot function - fGarch",
col = "palevioletred")
curve(expr = dnorm,
from = -5,
to = 5,
n = N_sim,
col = "limegreen",
lty = 2)

plot.function(x = function(t) dsn(x = t,
xi = central_tendency_sim,
omega = dispersion_sim,
alpha = skewness_sim),
to = 5,
from = -5,
n = N_sim,
xlab = "observations",
ylab = "densities",
main = "plot function - sn",
col = "skyblue")
curve(expr = dnorm,
from = -5,
to = 5,
n = N_sim,
col = "limegreen",
lty = 2)
`````` ``````
dev.off()
#> null device
#>           1
``````

Created on 2019-09-06 by the reprex package (v0.3.0)

3 Likes

Thanks @Yarnabrina a ton for helping me here.

Actually, I made a sketch to describe clearly and attached.
Black curve has peak at x = .2 but ranges from 0 - 1
Therefore, it got to be skewed to one side.

Actually, in those link, it is hardly understandable because there were no plots and just random script
Hard to even understand the solution.

No, Just a black one is enough.

If you want to generate a distribution that peaks near 0.2 and has most of its density between 0 and 1, the following call to dsn() from the sn package comes close.

``````library(sn)

X <- seq(-1, 2, 0.01)
plot(X, dsn(X, xi = 0.1, omega = 0.3, alpha = 5), type = "l")
abline(v = 0.2)
`````` Created on 2019-09-06 by the reprex package (v0.2.1)

2 Likes

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