One model in economics is the permanent income hypothesis, which basically states that a household's expenditures will not react to a change in income unless that change in income is viewed as being permanent

How would you use this model to predict the expenditure patterns over the course of a year of a real estate agent who only sells homes during the months of April through July?

The agent will not consume all of her income when it is earned. She knows that her paychecks will not be coming during August through March. As a result, some of her summer income will be saved for use during those months.