[R-sig-Geo] DCluster questions

James Rooney ROONEYJ4 at tcd.ie
Sun Sep 8 12:53:39 CEST 2013

Thanks for the reply Virgilio,

>> 2) Will opgam/kn.iscluster only detect "hotspots" or will they also detect
>> "coldspots" i.e. areas of statistically unlikely lower incidence rates ?

>You get a p-value, so you could take the areas with very large p-values
>as cold-spots. But detection of cold-spots is not of interest, is it?

Um I'm new to this field so I'm probably missing something but - why wouldn't it be interesting? It might tell me about local protective factors, or localized case ascertainment holes I would imagine ? Anyhow I have something in mind to do it - although its low down on my todo list for now.

>> 4) How do I decide what the correct value for fractpop is ? I initially had it set to .25 and I
>>was getting cluster of 50% of my cases which made no sense.

>You may try different values of fractpop if you want to detect smaller

Ok...but that feels kind of arbitrary to me. Is there a logic by which to select a size ?

>> 5) Is there any correction for multiple testing in the opgam() command ? I have over 3000 areas -
>> do I need to set a very low alpha ?

>For the Spatial Scan Statistic you should only pay attention to the
>significance of the most likely cluster, as this is what Kulldorff's
>test is testing for. But you can use the p-value reported for the other
>secondary  clusters as a guidance.

>For GAM, you probably want to make a multiple test correction. Another
>good option is to plot the centres of the clusters detected as this will
>give you and idea of the areas with significant high risk.

This reply confuses me - if I['m using the Kuldorff's scan stat inside the GAM - then I don't need to worry about multiple comparison or I do ?

>Hope this helps.


Yes - quite helpful and much appreciated. Thank you!


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