Are anesthetics safe? The National Halothane Study was a major investigation of the safety of anesthetics used in surgery. Records of over 850,000 operations performed in 34 major hospitals showed the following death rates for four common anesthetics:61 Anesthetic: A B C D Death rate: 1.7% 1.7% 3.4% 1.9%

There seems to be a clear association between the anesthetic used and the death rate of patients. Anesthetic C appears to be more dangerous.

(a) Explain why we call the National Halothane Study an observational study rather than an experiment, even though it compared the results of using different anesthetics in actual surgery.

(b) When the study looked at other variables that are related to a doctor’s choice of anesthetic, it found that Anesthetic C was not causing extra deaths.

Explain the concept of confounding in this context and identify a variable that might be confounded with the doctor’s choice of anesthetic.