Recent
News Stories about Shark Attacks in Hawaii

(Dec. 12, 2013)

(Dec. 12, 2013)

Some newspapers in Hawaii are reporting that, until recently, there are about 4 attacks per year of sharks on humans. In 2013, there have been 13 attacks so far. The writers of these stories think "something has changed", and then go on to discuss what and how to remediate.

If one assumes that rare events, like shark attacks, follow a Poissson distribution, the mean number of attacks is the same as the parameter Lamda of the distribution. Specifically, having a Poisson distribution with parameter Lambda means here that the probability of exactly K attacks in a year is then Exp(-Lambda) (Lambda^K)/(K!).

With the assumption that Lambda = 4 for shark attacks in Hawaii, having 13 shark attacks or more has probability of

0.000273717 (or about 3 chances in 10,000). So, each person has a choice: believe that 2013 is very unlucky year, or that the historical pattern of 4 shark attacks per year no longer is true.