As we did when considering only one variable, we begin with a graphical display. Ascatterplotis the most useful display technique for comparing two quantitative variables. We plot on the y-axis the variable we consider theresponsevariable and on the x-axis we place theexplanatory or predictorvariable.

How do we determine which variable is which? In general, the explanatory variable attempts to explain, or predict, the observed outcome. The response variable measures the outcome of a study. One may even consider exploring whether one variablecausesthe variation in another variable – for example, a popular research study is that taller people are more likely to receive higher salaries. In this case, Height would be the explanatory variable used to explain the variation in the response variable Salaries.

In summarizing the relationship between two quantitative variables, we need to consider:

Association/Direction (i.e. positive or negative)
Form (i.e. linear or non-linear)
Strength (weak, moderate, strong)