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Re: [ai-geostats] Using Jenks optimization in ArcMap - further significance tests required?

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  • Darla Munroe
    Hi Karen, One question - it sounds like you are plotting a categorical variable? Or do you have counts of that categorical variable? In any case, when you re
    Message 1 of 2 , Jan 27, 2005
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      Hi Karen,

      One question - it sounds like you are plotting a categorical variable? Or do you have counts of that categorical variable?

      In any case, when you're dealing with a variable for which you have no particular reason to think it should be randomly distributed (in terms of the function, NOT in space), then I always think some statistic that involves ranking (i.e., low to high) rather than value is a good one - so calculating observed and expected frequencies per area might be a good start, in your case. You possibly also could have spatial autocorrelation, so you might want to look for that (again, not knowing if your variable is continuous or categorical - there are a variety of techniques out there).

      But anyway, no, I would not say the Jenk's optimization method, though a useful visual tool, allows you to make any more than hypotheses about the variable's spatial distribution.

      Hope this helps,
      DM

      ----- Original Message -----
      From: Karen Bellinger Wehner <karen.wehner@...>
      Date: Thursday, January 27, 2005 2:13 am
      Subject: [ai-geostats] Using Jenks optimization in ArcMap - further significance tests required?

      > I am a graduate student doing a GIS archaeology project using
      > ArcMap
      > 8.1, and in need
      > of some statistical advice.
      >
      > Here's my question, and pardon me as I realize it betrays
      > frightening ignorance of statistics!
      >
      > Using the quantities-graduated colors option, I have plotted the
      > distribution of various artifact categories across 112 excavation
      > units within a fifty acre site. I
      > have done this in hopes of identifying areas with heightened
      > frequencies of specific artifact types, namely those involved in
      > craft
      > production, so as to identify previously unrecognized craft
      > activity
      > areas at the site (which was plowed and as a result, clearly
      > recognizable
      > features such as buildings have been destroyed - hence looking
      > merely
      > for concentrations of craft-related artifacts in what remains).
      >
      > I have selected the Jenks Natural Breaks/Optimization method
      > ArcMap
      > uses automatically in visualizing the plots using the color
      > gradations. I understand that Jenks classifies my data into
      > categories that maximize the difference between
      > categories and minimize the differences within categories.
      >
      > So here is the question: Does this process intersect with a chi-
      > square
      > or goodness of fit test? Does Jenks obviate the need for further
      > significance or "goodness of fit" testing? If not, would chi
      > square
      > be the way to go in assessing the significance of the observed
      > artifact distributions?
      >
      > ANY advice would be much appreciated.
      >
      > Cheers,
      >
      > Karen
      >
      > Karen Bellinger Wehner
      > PhD candidate
      > Anthropology Department
      > New York University
      >
      >
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