The relation between cause and effect is reversed.
Diet soda causes weight gain, or is it the other way around?
The Framingham analysis included 9,000 middle-aged men and women followed for four years. Researchers found that compared to people who didn’t drink sodas at all, those who drank both sugar-sweetened and diet soda were more likely to develop metabolic syndrome — a cluster of symptoms often linked to obesity that increase risk for heart disease and diabetes. Because both of these studies were observational, it is impossible to say if the diet sodas played a direct role in the weight gain. It may be that people switch to diet soda when they begin gaining weight without addressing other aspects of their diet that are causing the weight gain.Salynn Boyles, “Diet Sodas Cause Weight Gain? Not so Fast“, WebMD (November 29, 2010), emphasis added.
The increase in AIDS was caused by more sex education. (In fact, the increase in sex education was caused by the spread of AIDS.)
Give a causal argument showing that the relation between cause and effect has been reversed.
On scurrilous correlations
Linear regression is a marvelous tool, versatile, scalable, and as easy to execute as slicing a button on your spreadsheet. You can use it for data sets involving two variables, like the ones I’ve drawn here, but it works just as well for three variables