As an example, we’ll start with the normal distribution. As you may remember from statistics classes, the probability density function for the normal distribution is:

To find the probability density at a given value, use the `dnorm`

function:

dnorm(x, mean = 0, sd = 1, log = FALSE)

The arguments to this function are fairly intuitive: `x`

specifies the value at which to evaluate the
density, `mean`

specifies the mean of the
distribution, `sd`

specifies the standard
deviation, and `log`

specifies whether to
return the raw density (`log=FALSE`

) or
the logarithm of the density (`log=TRUE`

). As an example, you can plot the
normal distribution with the following command:

`> `**plot(dnorm, -3, 3, main = "Normal Distribution")**

The plot is shown in Figure 17-1.

Figure 17-1. Normal distribution

The distribution function for the normal distribution is `pnorm`

:

pnorm(q, mean = 0, sd = 1, lower.tail = TRUE, log.p = FALSE)

You can use the distribution function to tell you the probability
that a randomly selected value from the distribution is less than or equal
to *q*. Specifically, it returns *p*
= Pr(*x* ≤ *q*). The value
*q* is specified by the argument `q`

, the mean by `mean`

, and the standard deviation by `sd`

. If you would like the raw value
*p*, then specify `log.p=FALSE`

; if you would like
log(*p*), then specify `log.p=TRUE ...`

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