The status of the axillary lymph node basin is the most powerful predictor of long-term survival in patients with breast cancer. The pathologic analysis of the axillary nodes also provides essential information used to determine the administration of adjuvant therapies. Until recently, an axillary lymph node dissection (ALND) was the standard surgical procedure to identify nodal metastases. However, ALND is associated with numerous side effects, including arm numbness and pain, infection, and lymphedema.

A new procedure, sentinal lymp node (SLN) biopsy, has been proposed as a substitute and it has been reported to have a successful identification rate of about 90%. Suppose that we want to conduct a study to estimate and confirm this rate to identify nodal metastases among breast cancer patients because previous estimates were all based on rather small samples. How many patients are needed to confirm this 90% success rate with a margin of error of G5%? Does the answer change if we do not trust the 90% figure and calculate a conservative sample size estimate?