The Spanish patients, two men and two women, were elderly and very ill. All four had undergone routine dialysis at the Hospital de la Princesa. Soon after, they began sweating and vomiting, some of them in excruciating pain. They died within a few hours of receiving treatment.

Nothing seemed exceptional about the deaths of those old, sick patients on dialysis, even though four deaths had occurred in four days at the same place. Hospital officials noticed the manufacturers of the equipment involved, as dictated by protocol. But no one paid the incident much attention - until the following week, when people started dying in Valencia. " (Fast Company, 11/02)

QUESTION 1: At what point does the possibility that an event is more than coincidence become obvious? Consider this in terms of probability theory and statistical significance.