The research question below describes a relationship between two quantitative variables. Which variable should be plotted on the horizontal X-axis?

Is the amount of fat in a serving of cereal related to the amount of sugar in a serving?

fat in a serving

Incorrect. There is no logical explanatory-response relationship between these variables. We might expect fat and sugar to be associated, but either one could be used to predict the other. So either variable could be plotted on the X-axis.

sugar in a serving

either