英文:
Testing inhomogeneous patterns in spatstat
问题
我对我的数据应用了排列检验,以测试它们是否不均匀,见于《spatstatbook》第689页。作为青铜过滤器数据的示例。为此,我取消标记了我的点,并运行了两个测试(我还将我的区域划分为6个象限:2个20x150m和4个15x150m -> 总面积100x150m),结果显示我的总体数据更或多或少是均匀的(我通过kscaled和kinho行为的图像进行了测试,两者的行为几乎相同)。我的局部测试结果是 locTest(T=1.3437, p-value=0.225)
,corrTest(T=2.3059, p-value=0.052)
,这表明我的整体数据更或多或少是均匀的
。
尽管我已经取消标记我的数据进行分析,就像示例中一样,但我有很多标记(sp和许多功能性状)。我的问题是,我应该为每种标记
类型应用排列检验吗?对于分类的情况,是每个级别
都要进行吗?还是仅仅进行总体测试就足以满足均匀性的假设?
英文:
I applied the permutation test on my data to test if they are inhomogeneous, present on page. 689 of the spatstatbook. As an example of the bronze filter data. To do so, I unmarked my points and ran the two tests (I also divided my area into 6 quadrants: 2 of 20x150m and 4 of 15x150m -> total area 100x150m), which showed that my general data are more or less homogeneous (I did the test via image of the behaviors of kscaled and kinho, where both had practically the same behavior). My local tests gave locTest(T=1.3437, p-value=0.225)
, corrTest (T = 2.3059, p-value = 0.052)
, which concludes that my overall data is more or less homogeneous
.
Although I have unmarked
my data to do the analysis, as in the example, I have many marks (sp, and many functional traits). My question is, should I apply the permutation test for each mark
type? In the case of categorical, for each level
? Or does the general test alone suffice for the assumption of homogeneity?
答案1
得分: 2
这不是对《spatstat book》第689-694页描述的排列检验的正确解释。
如果你不确定一个点模式是否均匀,第一步应该是测试强度是否均匀,可以使用类似 quadrat.test
或 anova.ppm
的方法。
通常,当我们已经知道点模式不均匀,且尝试确定它表现出何种不均匀性时,会应用在第689-694页描述的 studpermu.test
函数。
确实,如果你在 studpermu.test
中使用了 summaryfunction=Kest
(默认设置),那么这将执行一个二阶均匀性的测试(更确切地说是检验不同子集的模式是否具有相同的K函数)。
然而,从你的问题中看,你似乎遵循了在第689-694页示例中使用的代码。该代码使用了 bronzefilter
数据集,这显然是不均匀的,问题是适用什么样的不均匀性假设。
你提到的 locTest
和 corTest
两个测试可能是通过分别使用 studpermu.test
并将 summaryfunction=Kscaled
和 summaryfunction=Kinhom
进行调用而进行的(遵循书中的代码)。对于 locTest
获得的 p-value 是0.225,这是不显著的,意味着接受局部缩放点过程的零假设。而对于 corTest
获得的 p-value 是0.052,非常接近于对相关性稳定过程零假设的正式拒绝。因此,这些测试表明你的未标记点模式来自一个不均匀、局部缩放的点过程 -- 不是均匀点过程。
英文:
This is not a correct interpretation of the permutation test described on pages 689-694 of the spatstat book.
If you're unsure whether a point pattern is homogeneous, the first step should be to test whether the intensity is homogeneous, using something like quadrat.test
or anova.ppm
.
Usually the function studpermu.test
described on pages 689-694 is applied when we already know that the point pattern is not homogeneous and we're trying to decide what kind of inhomogeneity it exhibits.
It's true that, if you used studpermu.test
with summaryfunction=Kest
(the default) then this would perform a test of second order homogeneity (more precisely - a test of whether the different subsets of the pattern have the same K-function).
However, from your question, it appears that you followed the code used in the example on pages 689-694. That code used the bronzefilter
dataset, which is very obviously inhomogeneous, and the question is what kind of inhomogeneity assumptions are appropriate.
The two tests locTest
and corTest
you mention were presumably carried out by calling studpermu.test
with summaryfunction=Kscaled
and summaryfunction=Kinhom
respectively (following the code in the book). The p-value that you get for locTest
is 0.225 which is not significant, meaning that the null hypothesis of a locally scaled point process is accepted. The p-value obtained for corTest
is 0.052 which is very close to a formal rejection of the null hypothesis of a correlation-stationary process. Therefore, these tests suggest that your unmarked point pattern comes from an inhomogeneous, locally-scaled point process -- not a homogeneous point process.
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