A state tourism board reports that 1.4 lakh visitors came to the district's limestone caves last year, the first full year after the board's advertisements for the caves began, and concludes that the advertisements brought the visitors to the caves. Which of the following is an assumption on which the board's conclusion depends?

A figure for the year after the advertisements shows what happened; it does not show what the advertisements did, and it cannot until it is set against what would have happened without them. If the caves drew 1.5 lakh in each of the three preceding years, the advertisements have cost money and lost visitors, and the board's conclusion is not merely unproved but false. The assumption is therefore precisely the missing comparison-point. The languages of the advertisements bear on their reach; where the visitors came from bears on the district's market; and who keeps the count bears on the figure's accuracy. Each of the three could be settled in the board's favour and the conclusion would still hang on a year that has never been weighed against any other.