 to give the new drug to the more severe cases, and the placebo to the less severe cases. Initial severity of disease is now <strong>confounded</strong> with the treatment, and any difference in outcome success observed between the experimental and control condition might be due to the treatment, or to the initial differences in severity. Or even to a third variable such as age, if it is itself related to both initial severity of disease as well as the effect of the drug. We wouldn’t be able to tell, and 