Directionality problem: a situation in which it is unclear which variable in an association came first. Third-variable problem: a situation in which plausible alternative explanations exist for the association between two variables. We must consider who the participants were and how they were selected. The size of the sample does not matter as much as the way the sample was selected from its population. Moderator: a third variable that, depending on its level, changes the relationship between the other two variables. Moderating variable example: marital status moderates the relationship between maternal employment and child achievement. When we are asking about moderators, our goal is to ask whether the association between the two variables is different within the levels of some third variable (the moderator). When we are asking about subgroups, our goal is to make sure that the overall association between the two variables is the same within the two subgroups.