In a study in UK, an association was found between sale of anti-arrhythmic drug and an increase in deaths due to asthma. This is an example of:-

Here we are looking at association between sale of a drug and increase in asthma, with no individual study of consumption of that drug & then getting asthma. Thus, it is Ecological study/ co-relational study. Ecological study is neither forward nor backward study, done at point of time. It can't calculate strength of association/ prevalence. It is based on Secondary data.