• Some observed associations between two variables are due to a cause-and-effect relationship between these variables, but others are explained by lurking variables.
• The effect of lurking variables can operate through common response if changes in both the explanatory and the response variables are caused by changes in lurking variables. Confounding of two variables (either explanatory or lurking variables or both) means that we cannot distinguish their effects on the response variable.
• Establishing that an association is due to causation is best accomplished by conducting an experiment that changes the explanatory variable while controlling other influences on the response.
• In the absence of experimental evidence, be cautious in accepting claims of causation. Good evidence of causation requires (1) a strong association, (2) that appears consistently in many studies, (3) that has higher doses associated with stronger responses, (4) with the alleged cause preceding the effect in time, and (5) that is plausible.