Response variable: measures an outcome of a study: dependent variable, depends on the explanatory variable. Explanatory variable: explain or influence changes in a response variable. In many studies, the goal is to show that changes in one or more explanatory variables actually cause changes in a response variable: many explanatory-response relationships do not involve direct causation. Ex. age and sex of a child can help predict future height but they do not cause a particular height. Most statistical studies examine data on more than one variable. Most useful graph for displaying the relationship between two quantitative variables = scatterplot. The strength of a relationship in a scatterplot is determined by how closely the points follow a clear form. Not all relationships have a simple form and a clear direction that we can describe as positive association or negative association. First describe the relationship between x and y separately for each category then compare and contrast these relationships.