On Jan 28, 2008, at 10:22 AM, Michael Barton wrote:
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?
I would like to add mean of absolute values of the attribute - this
is useful when the attribute is deviation or error
to measure accuracy of interpolation/approximation methods (there are
some papers that explain why MAE is
better than RMSE for this). Although it is not in the man pages,
v.univar computes it for the point option.
Writing out the range is a nice convenience too (see v.univar output
for points - it has pretty comprehensive
set of stats measures).
Helena
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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