# nncorr

##### Nearest-Neighbour Correlation Indices of Marked Point Pattern

Computes nearest-neighbour correlation indices of a marked point
pattern, including the nearest-neighbour mark product index
(default case of `nncorr`

),
the nearest-neighbour mark index (`nnmean`

),
and the nearest-neighbour variogram index (`nnvario`

).

- Keywords
- spatial, nonparametric

##### Usage

```
nncorr(X,
f = function(m1, m2) { m1 * m2 },
k = 1,
…,
use = "all.obs", method = c("pearson", "kendall", "spearman"),
denominator=NULL)
nnmean(X, k=1)
nnvario(X, k=1)
```

##### Arguments

- X
The observed point pattern. An object of class

`"ppp"`

.- f
Function \(f\) used in the definition of the nearest neighbour correlation. There is a sensible default that depends on the type of marks of

`X`

.- k
Integer. The

`k`

-th nearest neighbour of each point will be used.- …
Extra arguments passed to

`f`

.- use,method
Arguments passed to the standard correlation function

`cor`

.- denominator
Internal use only.

##### Details

The nearest neighbour correlation index \(\bar n_f\) of a marked point process \(X\) is a number measuring the dependence between the mark of a typical point and the mark of its nearest neighbour.

The command `nncorr`

computes the nearest neighbour correlation index
based on any test function `f`

provided by the user.
The default behaviour of `nncorr`

is to compute the
nearest neighbour mark product index.
The commands `nnmean`

and `nnvario`

are
convenient abbreviations for other special choices of `f`

.

In the default case, `nncorr(X)`

computes three different
versions of the nearest-neighbour correlation index:
the unnormalised, normalised, and classical correlations.

- unnormalised:
The

**unnormalised**nearest neighbour correlation (Stoyan and Stoyan, 1994, section 14.7) is defined as $$\bar n_f = E[f(M, M^\ast)]$$ where \(E[]\) denotes mean value, \(M\) is the mark attached to a typical point of the point process, and \(M^\ast\) is the mark attached to its nearest neighbour (i.e. the nearest other point of the point process).Here \(f\) is any function \(f(m_1,m_2)\) with two arguments which are possible marks of the pattern, and which returns a nonnegative real value. Common choices of \(f\) are: for continuous real-valued marks, $$f(m_1,m_2) = m_1 m_2$$ for discrete marks (multitype point patterns), $$f(m_1,m_2) = 1(m_1 = m_2)$$ and for marks taking values in \([0,2\pi)\), $$f(m_1,m_2) = \sin(m_1 - m_2)$$ For example, in the second case, the unnormalised nearest neighbour correlation \(\bar n_f\) equals the proportion of points in the pattern which have the same mark as their nearest neighbour.

Note that \(\bar n_f\) is not a ``correlation'' in the usual statistical sense. It can take values greater than 1.

- normalised:
We can define a

**normalised**nearest neighbour correlation by $$\bar m_f = \frac{E[f(M,M^\ast)]}{E[f(M,M')]}$$ where again \(M\) is the mark attached to a typical point, \(M^\ast\) is the mark attached to its nearest neighbour, and \(M'\) is an independent copy of \(M\) with the same distribution. This normalisation is also not a ``correlation'' in the usual statistical sense, but is normalised so that the value 1 suggests ``lack of correlation'': if the marks attached to the points of`X`

are independent and identically distributed, then \(\bar m_f = 1\). The interpretation of values larger or smaller than 1 depends on the choice of function \(f\).- classical:
Finally if the marks of

`X`

are real numbers, we can also compute the**classical**correlation, that is, the correlation coefficient of the two random variables \(M\) and \(M^\ast\). The classical correlation has a value between \(-1\) and \(1\). Values close to \(-1\) or \(1\) indicate strong dependence between the marks.

In the default case where `f`

is not given,
`nncorr(X)`

computes

If the marks of

`X`

are real numbers, the unnormalised and normalised versions of the nearest-neighbour product index \(E[M \, M^\ast]\), and the classical correlation between \(M\) and \(M^\ast\).If the marks of

`X`

are factor valued, the unnormalised and normalised versions of the nearest-neighbour equality index \(P[M = M^\ast]\).

The wrapper functions `nnmean`

and `nnvario`

compute the correlation indices for two special choices of the
function \(f(m_1,m_2)\).

`nnmean`

computes the correlation indices for \(f(m_1,m_2) = m_1\). The unnormalised index is simply the mean value of the mark of the neighbour of a typical point, \(E[M^\ast]\), while the normalised index is \(E[M^\ast]/E[M]\), the ratio of the mean mark of the neighbour of a typical point to the mean mark of a typical point.`nnvario`

computes the correlation indices for \(f(m_1,m_2) = (1/2) (m_1-m_2)^2\).

The argument `X`

must be a point pattern (object of class
`"ppp"`

) and must be a marked point pattern.
(The marks may be a data frame, containing several columns of mark variables;
each column is treated separately.)

If the argument `f`

is given, it
must be a function, accepting two arguments `m1`

and `m2`

which are vectors of equal length containing mark
values (of the same type as the marks of `X`

).
It must return a vector of numeric
values of the same length as `m1`

and `m2`

.
The values must be non-negative.

The arguments `use`

and `method`

control
the calculation of the classical correlation using `cor`

,
as explained in the help file for `cor`

.

Other arguments may be passed to `f`

through the `...`

argument.

This algorithm assumes that `X`

can be treated
as a realisation of a stationary (spatially homogeneous)
random spatial point process in the plane, observed through
a bounded window.
The window (which is specified in `X`

as `Window(X)`

)
may have arbitrary shape.
Biases due to edge effects are
treated using the ‘border method’ edge correction.

##### Value

Labelled vector of length 2 or 3 containing the unnormalised and normalised nearest neighbour correlations, and the classical correlation if appropriate. Alternatively a matrix with 2 or 3 rows, containing this information for each mark variable.

##### References

Stoyan, D. and Stoyan, H. (1994) Fractals, random shapes and point fields: methods of geometrical statistics. John Wiley and Sons.

##### Examples

```
# NOT RUN {
data(finpines)
nncorr(finpines)
# heights of neighbouring trees are slightly negatively correlated
data(amacrine)
nncorr(amacrine)
# neighbouring cells are usually of different type
# }
```

*Documentation reproduced from package spatstat, version 1.55-1, License: GPL (>= 2)*