Définition Ecological fallacy

'Ecological' in this case means 'collectively' and goes back to the city ecology of the Chicago school. The term 'ecological fallacy' is used when incorrect conclusions are drawn from aggregated data. Aggregated data are, other than individual data, based on an aggregation of data (e.g. average, percentage).

Let's suppose that there are 30% trade unionists living in an electoral district. At the next election, 30% of votes go to the Democrats. The tempting fallacy: unionists have all voted for the Democratic candidate. Of course, this need not necessarily be the case, from a statistical viewpoint, no interdependence of the data is ensured. It is equally possible that only half of the 30 percentage points came from trade unionists and the other half of union workers gave their votes to other (Republican) candidates.