Intro to validity: construct validity: concerns whether our methods of studying variables are accurate. Internal validity : refers to the accuracy of conclusions about cause and effect: external validity: concerns whether we can generalize the findings of a study to other settings. A variable is any event, situation, behaviour, or individual characteristic that varies. Each variable represents a general class within which specific instances will vary; these specific instances are called the levels or values of the variable. For example, a mediating variable would be diffusion of responsibility, which is when there are several bystanders, personal responsibility to help is diffused among all the bystanders, so no single person feels much responsibility. The operational definition of a variable is a definition of the variable in terms of the operations or techniques the researcher uses to measure or manipulate it. Variables must be operationally defined so they can be studied empirically.