Question: According to The Wellness Encyclopedia (University of California, 1991, p.17): "Alcohol consumed to excess increases the risk of cancer of the mouth, pharynx, esophagus, and larynx. These risks increase dramatically when alcohol is used in conjunction with tobacco." It is obviously not possible to conduct a designed experiment on humans to test this claim, so the causal conclusion must be based on observational studies. Explain three potential additional pieces of information that the authors may have used to lead them to make a causal conclusion.