Problem:
Consider the following statement: "Citizens who own an SUV are more likely to help their fellow citizens in a disaster than are citizens who own compact cars." What are the independent and the dependent variables, and what are some possible third variables? Be sure that the third variables you choose are plausible causes of variation in both the independent and dependent variables such that the third variable is a potential alternative explanation for the correlation of these two variables. Incidentally, the example is made-up and not based on data.
Switch your focus from identifying third variables to identifying additional variables. For example: "Households with children under 18 are more likely than households without children under 18 to evacuate when requested to do so." Identify the independent and the dependent variable. Now focus only on the dependent variable-evacuation-and offer other factors; that is, additional independent variables that might affect the likelihood of a household evacuating (e.g., presence of an extended family, access to transportation, pet ownership, area of town, etc.).