Sunday, May 5, 2013

Space-Time Cluster Analysis with SatScan

For more information: Visit the latest post on SatScan: http://opensourcegisblog.blogspot.com/2015/02/satscan-94-released-better-than-ever.html

Original post
Numerous basic and advanced techniques exist for finding spatial and temporal clusters.  Searching for clusters has broad applications for any field of scientific inquiry!

Unlike other spatial models in other free and paid software, SatScan's statistics' probability distributions allow for poisson (count data and rates) and binomial distributions--to name two.  There is also the ability to treat same data as continuous.  You won't find an easier way to do this than with SatScan!

SatScan is a free program but requires several steps to get data into it for analysis.  For most analyses you will need three files in a text delimited format -- without column headers (such as variable names).

The three files: 1)  A case file with a column for the geographic unit. day, month or year (see documentation), and number of cases.  You can aggregate the data into any geographic unit--large or small. 2) A geographic coordinate file (cartesian or lat/long) with the name of the unit (i.e. census tract), x and y for centroids of the geographic units, and 3) population file with the estimated population over the time period-- by year.

After this slightly painful process, which one learned, can easily be duplicated, one can easily perform complex spatial analysis and adjust key parameters such as the population at risk and maximum size of the cluster.  Time units are important, and you will have to make key decisions as to how long a cluster may have to develop--depending on the problem of interest.

SatScan can look for purely spatial, purely temporal, space-time, and spatial variation in temporal trends in data.   SatScan uses 'scan' statistics/scanning window and cylinder to finding and differentiating potential clusters.

SatScan's output includes *.txt and/or *.dbf files of the results and clusters.  The *.gis file can be joined to the shapefile of the geographic units, which are using, to show risks and different clusters.  This part is straightforward and less painful.  You will need to take your time selecting parameters and interepreting results!

Two good articles to read are: 1)  Block's Tutorial and Review   and 2) Visual Analytics of Space-Time Statistics.  The SatScan manual on its website also has a great list of references.

Additional Article:
http://medicine.plosjournals.org/archive/1549-1676/2/3/pdf/10.1371_journal.pmed.0020059-L.pdf

4 comments:

  1. Great post, could you list out other systems that work similar to the SaTScan, I want to know to far extent has this field been developed?

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  2. Hi Arjun,
    Scan statistics are very mature, and the best way to look at changes over space and time. Here is a list of papers using SatScan: http://www.satscan.org/references.html. Also, CrimeStat, a separate free program, has several other statistics: Knox and Mantel statistic. Read more at: http://goo.gl/4JaIIK. Lastly, ClusterSeer, a paid program, which I have not used, is apparently geared towards space-time analysis:http://www.biomedware.com/?module=Page&sID=clusterseer-overview.
    - Jon

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  3. is it possible to do space time clustering in Economic field?

    let say, i want to clustering regencies by inflation rate.

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