Tasting Soup, and How Not To
When you taste a spoonful of chicken soup and conclude that the whole pot needs salt, that's an inference. To know if your inference is valid, you have to know hoe your one spoonful "the data" got taken from the pot. If there's a lot of salt sitting on the bottom, soup from the surface won't be representative, and you'll end up with an incorrect inference. But if you stir the soup thoroughly before you taste, your spoonful of data can tell you about the whole pot. Obvious? Then why do surveys and polls sometimes go so wrong.