Our discount stores include Poundland, Poundsavers, Savers, Aladdins Cave, 99p shop, Netto - we are in a very downbeat part of the country, two of the most deprived wards in the country (as well as other far more well-off areas within a few miles). I work FT so don't have a lot of shopping time, but if I see a good offer advertised (e.g. coffee which is a brand I use) I will buy in these stores. I bought beer in Netto, OH's preferred brand, for £5 a pack less than in the supermarket. I do look for brand, we know what we like, but we always buy on offers anyway.
I wonder if your research could also consider the hypothesis of whether the discount stores offer better value for money consistently, rather than shoppers going to supermarkets (whose prices have come down) and buying reduced price damaged goods or short-dated items (which is how I do most of my shopping)?