This fallacy has the following general form:
The general idea behind this fallacy is that it is an error in reasoning to conclude that one thing causes another simply because the two are associated on a regular basis. More formally, this fallacy is committed when it is concluded that e1 is the cause of e2 simply because they are associated on a regular basis. The error being made is that a causal conclusion is being drawn from inadequate evidence.
The Questionable Cause Fallacy is actually a general type of fallacy. Any causal fallacy that involves an error in a reasoning due to a failure to adequately investigate the suspected cause is a fallacy of this type. Thus, fallacies like Post Hoc and Confusing Cause and Effect are specific examples of the general Questionable Cause Fallacy.
Causal reasoning can be quite difficult since causation is a rather complex philosophic issue. The complexity of causation is briefly discussed in the context of the specific versions of this fallacy.
The key to avoiding the Questionable Cause fallacy is to take due care in drawing causal conclusions. This requires taking steps to adequately investigate the phenomena in question as well using the proper methods of careful investigation.
Joe gets a chain letter that threatens him with dire consequences if he breaks the chain. He laughs at it and throws it in the garbage. On his way to work he slips and breaks his leg. When he gets back from the hospital he wsends out 200 copies of the chain letter, hoping to avoid further accidents.
When investigating a small pond a group of graduate students found that there was a severe drop in the fish population. Further investigation revealed that the fishes' food supply had also been severely reduced. At first the students believed that the lack of food was killing the fish, but then they realized they had to find what was causing the decline in the food supply. The students suspected acid rain was the cause of both the reduction in the fish population as well as the food supply. However, the local business council insisted that it was just the lack of food that caused the reduction in the fish population. Most of the townspeople agreed with this conclusion since it seemed pretty obvious that a lack of food would cause fish to die.