County building inspectors report that almost fifty percent of the homes they inspect are equipped with gauges that monitor carbon monoxide levels. Fifteen years ago, only twenty-five percent of inspected homes were equipped with these gauges. However, even though more homes are now monitored for poisonous carbon monoxide fumes, the total number of homes with confirmed dangerous fume levels is no higher now than it was fifteen years ago because a large proportion of gauges produce false readings.
Which one of the following assumptions must be made in order for the author to be correct in drawing the conclusion stated in the passage?