dclf.test
Diggle-Cressie-Loosmore-Ford and Maximum Absolute Deviation Tests
Perform the Diggle (1986) / Cressie (1991) / Loosmore and Ford (2006) test or the Maximum Absolute Deviation test for a spatial point pattern.
Usage
dclf.test(X, ..., rinterval = NULL, use.theo=FALSE)
mad.test(X, ..., rinterval = NULL, use.theo=FALSE)
Arguments
- X
- Either a point pattern (object of class
"ppp"
,"lpp"
or other class), a fitted point process model (object of class"ppm"
,"kppm"
or other class) or an envelope object (class"envelope"
- ...
- Arguments passed to
envelope
. Useful arguments includefun
to determine the summary function,nsim
to specify the number of Monte Carlo simulations, andverbose=
- rinterval
- Interval of values of the summary function argument
r
over which the maximum absolute deviation, or the integral, will be computed for the test. A numeric vector of length 2. - use.theo
- Logical value determining whether to compare the summary function
for the data to its theoretical value for CSR (
use.theo=TRUE
) or to the sample mean of simulations from CSR (use.theo=FALSE
).
Details
These functions perform hypothesis tests for goodness-of-fit of a point pattern dataset to a point process model, based on Monte Carlo simulation from the model.
dclf.test
performs the test advocated by Loosmore and Ford (2006)
which is also described in Diggle (1986), Cressie (1991, page 667, equation
(8.5.42)) and Diggle (2003, page 14).
mad.test
performs the
The type of test depends on the type of argument X
.
- If
X
is some kind of point pattern, then a test of Complete Spatial Randomness (CSR) will be performed. That is, the null hypothesis is that the point pattern is completely random. - If
X
is a fitted point process model, then a test of goodness-of-fit for the fitted model will be performed. The model object contains the data point pattern to which it was originally fitted. The null hypothesis is that the data point pattern is a realisation of the model. - If
X
is an envelope object generated byenvelope
, then it should have been generated withsavefuns=TRUE
orsavepatterns=TRUE
so that it contains simulation results. These simulations will be treated as realisations from the null hypothesis.
envelope
to
generate or extract the simulated summary functions.
The number of simulations that will be generated or extracted,
is determined by the argument nsim
, and defaults to 99.
The summary function that will be computed is determined by the
argument fun
(or the first unnamed argument in the list
...
) and defaults to Kest
(except when
X
is an envelope object generated with savefuns=TRUE
,
when these functions will be taken). The choice of summary function fun
affects the power of the
test. It is normally recommended to apply a variance-stabilising
transformation (Ripley, 1981). If you are using the $K$ function,
the normal practice is to replace this by the $L$ function
(Besag, 1977) computed by Lest
. If you are using
the $F$ or $G$ functions, the recommended practice is to apply
Fisher's variance-stabilising transformation
$\sin^{-1}\sqrt x$ using the argument
transform
. See the Examples.
Value
- An object of class
"htest"
. Printing this object gives a report on the result of the test. The $p$-value is contained in the componentp.value
.
References
Besag, J. (1977) Discussion of Dr Ripley's paper. Journal of the Royal Statistical Society, Series B, 39, 193--195. Cressie, N.A.C. (1991) Statistics for spatial data. John Wiley and Sons, 1991.
Diggle, P. J. (1986). Displaced amacrine cells in the retina of a rabbit : analysis of a bivariate spatial point pattern. J. Neuroscience Methods 18, 115--125. Diggle, P.J. (2003) Statistical analysis of spatial point patterns, Second edition. Arnold.
Loosmore, N.B. and Ford, E.D. (2006) Statistical inference using the G or K point pattern spatial statistics. Ecology 87, 1925--1931.
Ripley, B.D. (1977) Modelling spatial patterns (with discussion). Journal of the Royal Statistical Society, Series B, 39, 172 -- 212.
Ripley, B.D. (1981) Spatial statistics. John Wiley and Sons.
See Also
Examples
dclf.test(cells, Lest, nsim=39)
m <- mad.test(cells, Lest, verbose=FALSE, rinterval=c(0, 0.1), nsim=19)
m
# extract the p-value
m$p.value
# variance stabilised G function
dclf.test(cells, Gest, transform=expression(asin(sqrt(.))),
verbose=FALSE, nsim=19)