Mean absolute/unsigned error.

def mae(x_values, y_values, drop_missing = True):
    '''Mean absolute/unsigned error.'''
    num_points = len(x_values)
    assert(num_points == len(y_values) and num_points > 0)
    return numpy.sum(numpy.apply_along_axis(numpy.abs, 0, numpy.subtract(x_values, y_values))) / float(num_points)