[OSGeo Africa] Formulas for probablilities

[mmchari .]

Dear colleagues

I am trying to perform a vulnerability assessment using GIS for Nkonkobe
Local Municipality in Eastern Cape Province, South Africa. The.The
indicators to be used in assessment of exposure are:
Probability of extreme hot temperatures (A): Based on historical trends of
maximum temperature data (1980 – 2013).
Probability (in %) of meteorological drought (B): Based on long term
averages of rainfall data.
Probability (in %) of extreme cold temperatures (C): This will be based on
historical trends of minimum temperature data (1980 – 2013).

  Therefore from the above indicators, exposure is calculated using field
calculator from ArcGIS 10.1 as follows:

*                                    Exposure = (A+B+C) / 3*
Now about the formula for determining of  these probabilities. How can I
also get the probabilities in shapefile formats?

I would sincerely appreciate your assistance.

Martin
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