I have census block data polygons. I want to overlay them over Zip Codes (larger polygons) in order to estimate the population in the Zip Codes. Or overlay buffers around toxic waste sites over census blocks to estimate the population within the buffer. How is this done In QGIS?
Taking into account that all the comments (Ecological Fallacy and Mismatched Polygons inherent to combining ZipCode and Census Tract data), this would be my suggestion.
1) Estimate Population Density Using Census Tracts using Random Points.
2) Vector -> Analysis Tools -> Points in polygon