Suppose that a team of researchers is interested in the relationship between the price of a popular children's chocolate brand (let's call it "Jumbo Chocolate Egg") and its demand. The team conducts a series of surveys over a five-year period where they ask 200 households in a Vancouver neighborhood to report how many packs of Jumbo Chocolate Egg they bought in each quarter of the year. The company that produces Jumbo Chocolate Egg is interested in estimating the price elasticity of demand for their chocolate, so they changed the price of a pack of chocolate each quarter over this period. This survey is voluntary - the team went door-to-door to gather the data, and people could refuse to participate