During a prevalence study of HAIs, the following lengths of stay per infection were observed: 10, 4, 4, 1, 8, 7, 18, 25, 18, 9, 5, 18. What is the median length of stay?

• A. 6
• B. 7
• C. 8.5
• D. 9.5

Answer: (C)

The median is the value that splits the data in half. With an even number of observations, you average the two middle values. Sorting the lengths gives 1, 4, 4, 5, 7, 8, 9, 10, 18, 18, 18, 25. The two middle positions are the 6th and 7th values, which are 8 and 9. Averaging them gives (8 + 9) / 2 = 8.5. So the median length of stay is 8.5 days. In length-of-stay data, distributions are often skewed by a few long stays, so the median provides a better typical value than the mean. The other options would reflect picking one middle value or misidentifying the central pair, but the correct method uses the average of the two central values.