From TAKING SIDES: Clashing Views in Science and Technology:
Many cause/effect statements are flawed because no appropriate research or evidence has isolated a single cause. There may be other hidden factors underlying the relationship. A good example is this statement: "Birds fly south in winter because it gets cold in northern areas." This statement is plausible, and many readers would accept it because it "makes sense." Data exist to show a relationship between temperature and bird population density: population decreases as temperature decreases. However...

For Discussion:
  1. What are other possible underlying variables here?
  2. Can you think of experiments desined to eliminate some of the other variables?

However, no experiment has conclusively established that temperature is a causative factor of bird migration. Alternative hypotheses may very well also explain the behavior. Food supplies may become scarce during low-temperature periods, breeding instincts may precipitate migration, or the birds may simply want a change of scenery! If sufficiently controlled experiments could rule out these alternative hypotheses, the cause/effect statement could be made. As it is, a simple correlation (statement of coincidence) is all that remains.