suppose we are interested in studying the relationship between the shelf life of cheeses in a dairy factory and the thickness of the packaging material used for those cheeses. we would like to determine if there is a causal relationship between the thickness of the packaging material and the shelf life of the cheese; that is, does a change in the thickness of the packaging material cause a change in the shelf life of the cheese? select the study that would be best source of evidence for establishing the existence of a causal relationship.