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I have a project in QGIS where one of the things I need to do is work from a police data file with 9 months of crime information, linked to details of county population (age, employment, etc) to report the % of different crimes in each county (7 counties). When I follow the example screenshot given, word for word, to work out a %, I get an message which says: "An error occured while evaluating the calculation string.".

I'm not sure where i'm going wrong. Any guidence would be a lifesaver.

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Did you mean to attach a screenshot? – Simbamangu May 12 '12 at 17:08

The equation for the field calculator should simply be something like:


Where FIELDNAME_1 = the field with the number of a particular crime and FIELDNAME_2 = the field with the total number of crimes.

Remember that the data should be stored as numbers. If its as a string bad things will probably happen (possibly the reason for your error).

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