An illustration of a probability scale. It is a horizontal line: above the line, the left-hand end is labelled ‘Impossible’ and the right-hand end is labelled ‘Certain’; below the line, the left-hand end is labelled ‘0’, the midway point is labelled ‘’ and the right-hand end is labelled ‘1’. A cross labelled ‘(b)’ is about a quarter of the way along the line; a second cross, labelled ‘(a)’ and ‘(c)’, is at the right-hand end of the line.