I am trying to average all the data for one attribute in a particular census tract from the road segments data that lies inside of them. The documentation states that you can sum average etc. data by using join attributes by location. When I go into this feature it does not give me any option to choose sum or average the data. I select the attribute column I want to pass to the polygon layer to I can select the column I want to move to the layer, but not do any calculations. When I choose PM2 it just gives me a column in the new polygon layer labeled PM2 with null values. What am I missing?