So what I am working on is accessibility analysis, I want to know how many percent of the population of a country have access to certain health services within a given driving time. For example is it possible to visit a certain specialist within 45 mins driving time? So what I need to know is how much of the population falls within my drive time isochrones and how much outside of it.

I have my isochrone layers for driving distance and I have a Vector layer population grid (from Eurostat) where each population value is tied to a 1*1km cell. (I don´t know if this is important, but the data came in as a reference grid and a csv file I had to manually join to the grid by Grid ID). I tried using the "difference" vector processing tool in QGIS to substract the isochrone layer from my population grid layer. What I got out looks exactly how I wanted it to look, it shows me just the population which falls outside of the isochrones. However, the attribute table stayed the same, it has exactly as many entries as before so it is useless for calculations. I want to compare the underserved population to the overall population.

Do you know how to perform a similar difference operation on my grid that will also remove the connected attributes to the removed grids?

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