Calculating statistics

In GIS, statistics can be computed from both raster and vector data. However, even calculating raster statistics often involves some kind of vector data. For example, we would like to include some statistical indices in our assessment regarding the suitable areas. More precisely, we would like to include at least the minimum, maximum, and average slope, the minimum, maximum, and average suitability, the average distance from the mass point of the settlements, and the minimum distance from waters. For this task, we cannot use our rasters alone; we need to calculate indices from them only where they overlap with our suitable areas. For this, we need our suitable areas as polygons, and then we can leave the rest of the work to QGIS.

In order to get our suitable areas as polygons, we need to delimit them on our suitability layer. The most trivial first choice is to select every cell with an excellent rating. However, how many cells do we have with more than 75% suitability...