On Jan 28, 2008, at 5:50 AM, Moritz Lennert wrote:
On 27/01/08 20:30, Michael Barton wrote:
v.univar only works with points. But since it is calculating stats
on a field in the attributes table, it should work the same for
all vector objects. Can we get rid of the limitation that it only
works with points?
There was some debate [1] about the statistical validity of working
with the other types, as the way it was programmed, the statistics
were calculated with weights which corresponded to line length /
area surface .
I guess we might want to distinguish between a v.univar which works
on the actual vector objects from a v.db.univar which works on any
arbitrary attribute (or combination of attributes). We could write
a C-replacement of the current v.db.univar script on the base of
the code I have for the classification algorithms used in v.class.
AFAICT, v.univar does not calculate anything from vector topology,
only from an attribute column. That is, it behaves the way you
describe v.db.univar. For some weird (probably historical) reason, it
won't calculate anything but N, max, and min of an attribute column
linked to a non-point vector object.
"v.univar calculates univariate statistics of vector map features.
This includes the number of features counted, minimum and maximum
values, and range. Variance and standard deviation is calculated only
for points if type=point is defined.
Extended statistics adds median, 1st and 3rd quartiles, and 90th
percentile."
An attribute is the same whether it's linked to a point, line, or area.
It would be nice to be able to calculate some stats from topology,
but that is not possible at the moment without loading topology.
As mentioned earlier, it might be better that I move the code from
v.class into a library which can then be accessed by different
modules...
Currently, I have defined the following statistics:
typedef struct
{
double count;
double min;
double max;
double sum;
double sumsq;
double mean;
double var;
double stdev;
} STATS;
But this could easily be extended according to needs and
v.db.univar could also use the quantile classification algorithm to
extract percentiles.
What are the statistics most people need ?
median, mode, and percentiles would be nice for any attribute or
topological data. Any diversity or other non-parametric stats that
would actually be useful here?
It would also be nice to get v.report type information (count,
length, area) summed by value of a string attribute column or binned
numberic column. But this might better be an extension of v.report.
Thanks for checking into it.
Michael
Moritz
[1] http://lists.osgeo.org/pipermail/grass-dev/2004-July/014976.html
____________________
C. Michael Barton, Professor of Anthropology
Director of Graduate Studies
School of Human Evolution & Social Change
Center for Social Dynamics & Complexity
Arizona State University
Phone: 480-965-6262
Fax: 480-965-7671
www: <www.public.asu.edu/~cmbarton>
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