According to Alex Tabarrok at Marginal Revolution, a scientific sampling of people who died in the last year would show that reading Harry Potter novels is strongly correlated with dying young. If you don't read Harry Potter you're much more likely to live to a ripe old age. Wish I had known that before I read the series.
The point is that the sample would be skewed. A proper sample would be one of people who read Harry Potter.
Just a reminder of the fallacy traps present in using statistics.