Five values and the median
Five independent observations are drawn from the same continuous distribution, whose median is .
Find the probability that lies between the smallest and the largest of the five observations.
Fractions, decimals with at least 3 significant digits, and expressions such as sqrt(2)/2, e^-1 or pi/4.
When does fail to lie between the smallest and the largest?
Each observation is below with probability , whatever the distribution.
h
s Shown after a correct answer. Revealing it first marks the problem as seen, not solved.
Idea. The median is outside the range of the sample only if all five observations fall on the same side of it. Each falls below the median with probability exactly , so the distribution does not matter.
Step 1. is below the smallest observation exactly when all five are above : probability . Likewise, is above the largest when all five are below : also . (Since the distribution is continuous, no observation equals .)
Step 2.
Why it matters. With only five data points, the interval from the smallest to the largest is already a 94% confidence interval for the median, with no assumption on the shape of the distribution. With points the confidence is .