Stores

The Open Food Facts data base gathers essentially products from France and United States. Therefore, in order to compare the results, we concentrated our study on France stores. French stores and supermarket have different reputations. Some like Lidl or Aldi are very cheap and discount stores whereas Monoprix is supposed to be a high quality super market. Some others like Carrefour or Leclerc are intermediate superior quality supermarkets.
Question is: if the difference of reputation is somewhat obvious on the prices of the products in those stores, does a significant difference also exists on the quality of the food?

Hierarchy of stores

Just like for the comparison between organic and non-organic food, the nutriscore is used to compare the quality of the food sold by the stores. The lower the nutriscore, the higher the quality of the food.

Comparison of nutriscores between stores.

Only the top 10 represented stores in the Open Food Facts data base are represented in order to have a clarity of the graphs and to compare stores with comparable number of products referenced.
On this chart, there appears to be some differences between the mean nutriscore of the stores. However, the variance for a given store is quite high given the multiplicity of categories of products sold. If these differences are significant, could we create a hierarchy of stores and sort them into quality levels?

Levels of quality

p-values of the t-test between the distributions of nutriscores for each store.

By considering a threshold of 0.01, we can construct a graph linking stores with a not substantially different distribution of nutriscores. That way, we are able to cluster them.

Graph of equivalent stores.

Low-cost low quality food.

High quality food. (An outlier not selling the same products as the others)

More expensive food of higher quality.

These charts show an interesting fact: the stores that have indeed a reputation of being discount stores (Aldi or Lidl) indeed offer products of lower quality. On the other hand, stores like Carrefour or Franprix which are somewhat more expensive have a healthier offer of products. The only outlier is Picard that has a very different panel of products on sale. It is indeed a store specialized in frozen products. The fact that most of those are frozen vegetables, meat or fish and very few transformed food can explain the fact that the nutriscore is quite lower. Those products are indeed produced with less sugars and oil.