A 65-year-old man with a history of congestive heart failure is hospitalized with chest pain and hypotension requiring admission to the cardiac care unit. An intra-arterial line is placed for direct blood pressure monitoring. Consecutive readings of his intra-arterial blood pressure are 75, 110, 80, 90, 75, and 110 mm Hg. Which of the following represents the median of these blood pressure readings?

Question

A 65-year-old man with a history of congestive heart failure is hospitalized with chest pain and hypotension requiring admission to the cardiac care unit.  An intra-arterial line is placed for direct blood pressure monitoring.  Consecutive readings of his intra-arterial blood pressure are 75, 110, 80, 90, 75, and 110 mm Hg.  Which of the following represents the median of these blood pressure readings?

Quizplus · Accepted Answer

It is important to understand the difference between the median, mean, and mode, which are 3 measures of central tendency. The median of an ordered dataset is the number that separates the right half of the data from the left half. The dataset MUST be ordered before the median is determined. If the number of observations in the dataset is even, finding the median requires adding the middle 2 values together and dividing that sum by 2. The ordered dataset in this case is {75, 75, 80, 90, 110, 110}. There are 6 observations, which is an even number of observations. The median value splits the dataset in half; it lies between 80 and 90 (3 values on the left and 3 values on the right). The median is (80 + 90) / 2 = 85 mm Hg. Now, assume one of the values is missing and the ordered dataset includes the following 5 observations (odd number of observations): {75, 80, 90, 110, 110}. In this case, the median value would be 90 mm Hg, which splits the dataset in half (2 values on the left and 2 values on the right). (Choices A and D) The values 80 and 100 mm Hg are not measures of the center of the dataset. (Choice C) To find the mean of a dataset, add all of the observations and divide that sum by the number of observations. In this case, the mean is (75 + 75 + 80 + 90 + 110 + 110) / 6 = 90 mm Hg. (Choice E) Another measure of the center of a dataset is the mode. Finding the mode is easy because it is the most frequent value of a dataset. In our scenario, the dataset is "bimodal" because 75 and 110 mm Hg are each listed twice. Educational objective: The median is the value that is located in the precise center of an ordered dataset. It separates the right half of the data from the left half.

A 65-Year-Old Man with a History of Congestive Heart Failure