Actually, it's not very clear to me why you would want an inverted index for this. Inverted indexes are good for searching data that is NOT already organized into a structure. Typically, it would be for full-text searching. To answer questions like "how many responents chose answer C on question 6?" you would be better off with a standard databases. MySQL can handle tables with many millions of rows in them, so your data set doesn't sound like a problem to me at all.