# Chisquare

##### The (non-central) Chi-Squared Distribution

Density, distribution function, quantile function and random
generation for the chi-squared (\(\chi^2\)) distribution with
`df`

degrees of freedom and optional non-centrality parameter
`ncp`

.

- Keywords
- distribution

##### Usage

```
dchisq(x, df, ncp = 0, log = FALSE)
pchisq(q, df, ncp = 0, lower.tail = TRUE, log.p = FALSE)
qchisq(p, df, ncp = 0, lower.tail = TRUE, log.p = FALSE)
rchisq(n, df, ncp = 0)
```

##### Arguments

- x, q
- vector of quantiles.
- p
- vector of probabilities.
- n
- number of observations. If
`length(n) > 1`

, the length is taken to be the number required. - df
- degrees of freedom (non-negative, but can be non-integer).
- ncp
- non-centrality parameter (non-negative).
- log, log.p
- logical; if TRUE, probabilities p are given as log(p).
- lower.tail
- logical; if TRUE (default), probabilities are \(P[X \le x]\), otherwise, \(P[X > x]\).

##### Details

The chi-squared distribution with `df`

\(= n \ge 0\)
degrees of freedom has density
$$f_n(x) = \frac{1}{{2}^{n/2} \Gamma (n/2)} {x}^{n/2-1} {e}^{-x/2}$$
for \(x > 0\). The mean and variance are \(n\) and \(2n\). The non-central chi-squared distribution with `df`

\(= n\)
degrees of freedom and non-centrality parameter `ncp`

\(= \lambda\) has density
$$
f(x) = e^{-\lambda / 2}
\sum_{r=0}^\infty \frac{(\lambda/2)^r}{r!}\, f_{n + 2r}(x)$$
for \(x \ge 0\). For integer \(n\), this is the distribution of
the sum of squares of \(n\) normals each with variance one,
\(\lambda\) being the sum of squares of the normal means; further, \(E(X) = n + \lambda\), \(Var(X) = 2(n + 2*\lambda)\), and
\(E((X - E(X))^3) = 8(n + 3*\lambda)\). Note that the degrees of freedom `df`

\(= n\), can be
non-integer, and also \(n = 0\) which is relevant for
non-centrality \(\lambda > 0\),
see Johnson *et al* (1995, chapter 29).
In that (noncentral, zero df) case, the distribution is a mixture of a
point mass at \(x = 0\) (of size `pchisq(0, df=0, ncp=ncp)`

and
a continuous part, and `dchisq()`

is *not* a density with
respect to that mixture measure but rather the limit of the density
for \(df \to 0\). Note that `ncp`

values larger than about 1e5 may give inaccurate
results with many warnings for `pchisq`

and `qchisq`

.

##### Value

`dchisq`

gives the density, `pchisq`

gives the distribution
function, `qchisq`

gives the quantile function, and `rchisq`

generates random deviates. Invalid arguments will result in return value `NaN`

, with a warning. The length of the result is determined by `n`

for
`rchisq`

, and is the maximum of the lengths of the
numerical arguments for the other functions. The numerical arguments other than `n`

are recycled to the
length of the result. Only the first elements of the logical
arguments are used.

##### Note

Supplying `ncp = 0`

uses the algorithm for the non-central
distribution, which is not the same algorithm used if `ncp`

is
omitted. This is to give consistent behaviour in extreme cases with
values of `ncp`

very near zero. The code for non-zero `ncp`

is principally intended to be used
for moderate values of `ncp`

: it will not be highly accurate,
especially in the tails, for large values.

##### References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
*The New S Language*.
Wadsworth & Brooks/Cole. Johnson, N. L., Kotz, S. and Balakrishnan, N. (1995)
*Continuous Univariate Distributions*, chapters 18 (volume 1)
and 29 (volume 2). Wiley, New York.

##### See Also

Distributions for other standard distributions. A central chi-squared distribution with \(n\) degrees of freedom
is the same as a Gamma distribution with `shape`

\(\alpha =
n/2\) and `scale`

\(\sigma = 2\). Hence, see
`dgamma`

for the Gamma distribution.

##### Examples

`library(stats)`

```
require(graphics)
dchisq(1, df = 1:3)
pchisq(1, df = 3)
pchisq(1, df = 3, ncp = 0:4) # includes the above
x <- 1:10
## Chi-squared(df = 2) is a special exponential distribution
all.equal(dchisq(x, df = 2), dexp(x, 1/2))
all.equal(pchisq(x, df = 2), pexp(x, 1/2))
## non-central RNG -- df = 0 with ncp > 0: Z0 has point mass at 0!
Z0 <- rchisq(100, df = 0, ncp = 2.)
graphics::stem(Z0)
## visual testing
## do P-P plots for 1000 points at various degrees of freedom
L <- 1.2; n <- 1000; pp <- ppoints(n)
op <- par(mfrow = c(3,3), mar = c(3,3,1,1)+.1, mgp = c(1.5,.6,0),
oma = c(0,0,3,0))
for(df in 2^(4*rnorm(9))) {
plot(pp, sort(pchisq(rr <- rchisq(n, df = df, ncp = L), df = df, ncp = L)),
ylab = "pchisq(rchisq(.),.)", pch = ".")
mtext(paste("df = ", formatC(df, digits = 4)), line = -2, adj = 0.05)
abline(0, 1, col = 2)
}
mtext(expression("P-P plots : Noncentral "*
chi^2 *"(n=1000, df=X, ncp= 1.2)"),
cex = 1.5, font = 2, outer = TRUE)
par(op)
## "analytical" test
lam <- seq(0, 100, by = .25)
p00 <- pchisq(0, df = 0, ncp = lam)
p.0 <- pchisq(1e-300, df = 0, ncp = lam)
stopifnot(all.equal(p00, exp(-lam/2)),
all.equal(p.0, exp(-lam/2)))
```

*Documentation reproduced from package stats, version 3.3.3, License: Part of R 3.3.3*