Our side-by-side comparisons can be influenced by extreme possibilities such as extravagant wines and dilapidated houses, but they can also be influenced by the addition of extra possibilities that are identical to those we are already considering.  For example, in one study, physicians read about Medication X and were then asked whether they would prescribe the medication for a patient with osteoarthritis.  The physicians clearly considered the medication worthwhile, because only 28 percent chose not to prescribe it.  But when another group of physicians was asked whether they would prescribe Medication X or an equally effective Medication Y for a patient with the same disease, 48 percent chose to prescribe nothing.  Apparently, adding another equally effective medication to the list of possibilities made it difficult for the physicians to decide between the two medications, thus leading many of them to recommend neither. (p. 142)