Select the sample most representative of the population of interest.
A researcher wants to determine the relative merits of oak and maple as firewood.
◦ Thirty friends, half of whom burn only oak, half of whom burn only maple
◦ Fifteen pieces of each type of wood cut at the same time from branches of the same size
◦ Fifteen pieces of each picked up from the floor of a hardwood forest
◦ Fifteen pieces of each obtained from friends with woodpiles