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.