"""Running statistics — contains a bug in `median` for even-length inputs.

The bug: for an even number of elements, `median` returns the lower-middle element
instead of the average of the two middle elements. `mean` is correct.
"""


def mean(xs):
    return sum(xs) / len(xs)


def median(xs):
    s = sorted(xs)
    n = len(s)
    mid = n // 2
    return s[mid]  # BUG: even-length should average s[mid-1] and s[mid]