Medical care has come a long way in the last century. People live longer; deadly diseases are prevented, cured, or ameliorated. In short, medical care is more effective than at any previous time. In this sense, effectiveness is a straightforward concept that measures changes in life expectancy, quality of life, and occurrence of disease. Effectiveness is sometimes difficult to quantitate; yet it is an important part of clinical decision making. For example, is the use of universal precautions effective in preventing the spread of the human immunodeficiency virus? Is antibiotic prophylaxis effective in preventing postoperative wound infections? Is chest percussion effective in the therapy of' pneumonia? Is a brain biopsy more effective than empiric therapy in treating encephalitis?