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Yoroku: The danger of tsunamis in Japan and the difficult of evaluating probabilities

In light of recent announcements in Japan, some may find food for thought in the following scenario, which, we should point out, is merely hypothetical:

An infectious disease is expected to kill 600 people, and there are two health programs to choose from. If program A is implemented, it will save 200 people. If program B is adopted there is a one-third chance that all 600 will be saved and a two-thirds chance that none of them will be saved. When psychologists asked a subject group which alternative they would choose, 72% of respondents chose program A.

A second group was told that program A would result in 400 people dying, while under program B there was a one-third probability that no one would die, and a two-thirds chance that all 600 would die. When presented with these options, 78% of the subjects chose program B.

Looking at the two sets of problems, it is evident the survival rates for both programs do not change; the questions are merely phrased to place an emphasis on life or death.

This study is outlined in the book, "The Trouble with Science," by Robin Dunbar. The book cites the finding by psychologists that "people often have surprising difficulty interpreting probabilities."

Turning our attention to Japan, it was recently announced that that there was a 26% or higher chance of waves measuring at least 5 meters high hitting the coast of 29 municipalities within the next 30 years as a result of a Nankai Trough quake. These include municipalities in Shizuoka, Wakayama and Kochi prefectures on Japan's east coast. Meanwhile, the probability of waves measuring 10 meters or higher striking the coast of 21 municipalities ranged from 6-26%. A probability of 26% within 30 years is said to be the equivalent of once in a hundred years.

Eight years ago, people gulped with apprehension when they were told that a Nankai Trough megaquake could produce a tsunami up to 34 meters high in a worst-case scenario. This time, however, the scenario was not the very biggest type of quake that could kill hundreds of thousands of people, but the probabilities of tsunamis that posed a more imminent threat.

Nevertheless, it doesn't feel as if the danger has become more imminent. Perplexed residents and local bodies no doubt want to ask about the significance of the probability data and how it should be put to use in disaster mitigation.

Noting that people have a fundamental problem interpreting probabilities, the aforementioned book concludes, "Humans were not designed by evolution to evaluate probabilities carefully."

("Yoroku," a front-page column in the Mainichi Shimbun)

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