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| GeoCommunity Mailing List |
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| Mailing List Archives |
| Subject: | Re: GISList: Assigning Percentiles in ArcView? |
| Date: |
02/13/2002 11:11:15 PM |
| From: |
Quantitative Decisions |
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At 02:36 PM 2/13/02 -0500, Glenn Bigler wrote:
>I'd like to rank block groups in a particular market by a certain
>demographic value. Anyone know of an Avenue script or any other options?
For percentiles--the subject of the query--the easy way is to display the
block groups in a view as a theme, edit the legend to show the demographic
value, and choose a quantile classification. The legend classes will then
provide the quantiles and the map will symbolize block group
quantiles. You can use practically any quantile you like, even though the
classification sub-dialog appears to max out at 64 classes: just type over
the number. Use 100, for instance, to get percentiles.
For ranking, you don't need percentiles: just sort the attribute
table. Permanent sorting is available through the free "sort and promote"
script at http://www.quantdec.com/arcview1.htm#Legends. Use the Field
Calculator to assign ranks to the permanently sorted records with the
expression
rec+1
If you like, use the Field Calculator to convert these ranks to percentiles
(here we come full circle). There are many formulas for percentiles: they
and the principle behind them are discussed on the web at
http://www.quantdec.com/envstats/notes/class_02/characterizing_distributions.htm#percentiles
(this is part of a set of course notes on environmental statistics, which
may be of interest to some GIS people, regardless of what software they
use). An Excel spreadsheet and VBA macro are also available from this
page, which provides yet another way to obtain the percentiles: edit the
feature table in Excel (carefully: see
http://www.quantdec.com/SYSEN597/GTKAV/section4/chapter_15a.htm#Recipe for
how to do it without clobbering your data).
Although I have assumed that your data are in shapefile format, if they are
not then they are readily converted: use the Theme|Save as shapefile option
in the View menu if need be.
The two best places to get answers to ArcView-specific questions are the
ArcView-L and ArcView list servers. Learn about them or search their
archives (simultaneously!) at www.directionsmag.com/discussion . The
ArcView-L list always has a lot of newbies rotating through and so is a
good place for basic questions. The ArcView list has many experts and is a
good place for tough or unusual questions.
Cheers,
Bill Huber
Quantitative Decisions
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