A ward member urges that the ward's lanes be laid in concrete rather than in bitumen, and offers as his ground a note of the state cement dealers' association which finds concrete the cheaper surface over thirty years. Which of the following, if true, most weakens the ward member's reliance on that note?

The ward member is leaning on the note's authority, and authority is worth leaning on only where the writer had nothing to gain by the answer or where his working can be checked. Here the writer's income rises with the answer he gave and no working is shown, so nothing is left to lean on — though concrete may still, for all this shows, be the cheaper surface. Length and breadth of citation are not independence; when the lanes were last surfaced bears on the need for work, not on whose figures to trust; and how bitumen is procured does not touch the note's reliability.