Some doctors believe that a certain drug reduces the duration of episodes of vertigo, claiming that the average duration of vertigo for people who suffer from it has decreased since the drug was introduced  However, during a recent three-month shortage of the drug, there was no significant change in the average duration of vertigo
Thus, we can conclude that the drug has no effect on the duration of vertigo
If there were any change in the average duration of vertigo since the introduction of the drug, it would have demonstrated that the drug has an effect on the duration of vertigo.