help for ^spautoc5^

Spatial autocorrelation (Moran and Geary measures) --------------------------------------------------

^spautoc5^ valuesvar neighboursvar [^if^ exp] [^in^ range] [, ^w(^strvar^) lmea^n^(^newvar^) lmed^ian^(^newvar^)^ ] Description -----------

^spautoc5^ calculates Moran and Geary measures of spatial autocorrelation for a spatial variable valuesvar and neighbourhood information given by neighboursvar.

Remarks ------- For n values of a spatial variable x defined for various locations, which might be points or areas, calculate the deviations _ z = x - x

and for pairs of locations i and j, define a matrix

W = ( w ) ij describing which locations are neighbours in some precise sense. For example, w might be assigned 1 if i and j are contiguous areas ij and 0 otherwise; or w might be a function of the distance between ij i and j and/or the length of boundary shared by i and j.

The Moran measure of autocorrelation is

n n n n n 2 n ( SUM SUM z w z ) / ( 2 (SUM SUM w ) SUM z ) i=1 j=1 i ij j i=1 j=1 ij i=1 i

and the Geary measure of autocorrelation is

n n 2 n n n 2 (n -1) ( SUM SUM w (z - z ) ) / ( 4 (SUM SUM w ) SUM z ) i=1 j=1 ij i j i=1 j=1 ij i=1 i

and these measures may used to test the null hypothesis of no spatial autocorrelation, using both a sampling distribution assuming that x is normally distributed and a sampling distribution assuming randomisation, that is, we treat the data as one of n! assignments of the n values to the n locations.

^spautoc5^ avoids the use of Stata's matrix language, to avoid the limit of a maximum of 800 locations which that would imply, and to avoid the need to handle a matrix that in many problems would be very sparse. The price paid is a data structure for the neighbourhood information that is idiosyncratic by Stata standards.

In a toy example, area 1 neighbours 2, 3 and 4 and has value 3 2 1 and 4 2 3 1 and 4 2 4 1, 2 and 3 1 This would be matched by the data

^_n^ (obs no) ^value^ (numeric variable) ^nabors^ (string variable) ----------- ------------------------ ------------------------ 1 3 "2 3 4" 2 2 "1 4" 3 2 "1 4" 4 1 "1 2 3"

That is, ^nabors^ contains the observation numbers of the neighbours of the location in the current observation, separated by spaces. Therefore, the data must be in precisely this sort order when ^spautoc5^ is called.

Note various assumptions made here:

1. The neighbourhood information can be fitted into at most a ^str80^ variable.

2. If i neighbours j, then j also neighbours i and both facts are specified.

By default this data structure implies that those locations listed have weights in W that are 1, while all other pairs of locations are not neighbours and have weights in W that are 0.

If the weights in W are not binary (1 or 0), use the ^weights^ option. The variable specified must be another string variable.

^_n^ (obs no) ^nabors^ (string variable) ^weight^ (string variable) ----------- ------------------------ ------------------------ 1 "2 3 4" ".1234 .5678 .9012" etc.

that is, w = 0.1234, and so forth. w need not equal w . 12 ij ji Again, the assumption here is that these weights can be fitted into at most a ^str80^ variable.

Options -------

^w(^strvar^)^ specifies a string variable containing numeric weights, as explained above.

^lmean(^newvar^)^ and ^lmedian(^newvar^)^ specify that new variables should be generated containing local means and local medians, that is, means and medians of the neighbours of each location (not including that location). Any weights specified will be used in the calculation.

Examples --------

. ^spautoc5 cows nabors^

References ----------

Cliff, A.D. and Ord, J.K. 1973. Spatial autocorrelation. London: Pion.

Cliff, A.D. and Ord, J.K. 1981. Spatial processes: models and applications. London: Pion.

Author ------

Nicholas J. Cox, University of Durham, U.K. n.j.cox@@durham.ac.uk

Also see ---------

On-line: help for @dupneigh5@, @neigh5@, @numids5@