The board of directors of a city- owned electric power plant in a large urban city wants to assess the increase in electricity demands due to sources such as hybrid cars, big- screen TVs, and other entertainment devices in the home. There are a number of different sampling plans that can be implemented to survey the residents of the city. What are the relative merits of the following sampling units: individual families, dwelling units (single- family homes, apartment buildings, etc.), and city blocks?