A mill's yearly note states: 'The new boiler was fitted on 20 December. Coal burnt in the year just ended was 9 in every hundred below the coal burnt in the year before. The new boiler has therefore saved the mill 9 in every hundred of its coal.' The note's reasoning is most open to criticism because it

For three hundred and fifty-four days of the year in question the old boiler was burning the coal, so at most eleven days of the saving can belong to the new one; whatever cut the other eleven-twelfths must be looked for elsewhere. The cost of the boiler bears on whether it was worth fitting, not on what it saved. Whether other fuels are burnt does not affect a claim made about coal alone, and a share and a tonnage are two ways of stating the same fall.