Simple arithmetic produces some faulty reasoning

To the editor:

Several of your readers probably have noted the rather serious reasoning error in the Aug. 17 letter in which the writer incorrectly estimates the number of potential terrorists in this country holding student visas as follows: One out of 19 of the Sept. 11 terrorists was on a student visa, and there were 547,000 international students in the country at the time. So he estimates there are 28,789 (547,000 divided by 19) potential terrorists masquerading as students.

Here is the identical argument, substituting different groups: One out of the 19 most popular radio talk-show hosts in the nation is from Cape Girardeau, and the population of Cape Girardeau is 35,349. So there are 1,860 (35,349 divided by 19) potential nationally popular radio talk-show hosts within the city.

In both cases, the reasoning is simply incorrect. The laws of proportion which we learned long before high school have been misapplied. The proportion "one out of 19" student-visa holders among Sept. 11 terrorists (or Cape natives among national talk-show hosts) has been reversed to estimate terrorists out of student visa holders (or talk-show hosts among Cape natives.) From the data given, it is simply not possible to estimate the proportion of potential terrorists among student visa holders.

The war on terrorism certainly calls for our very best efforts. For us ordinary citizens, this means the best possible clear, logical thinking on issues of public concern.

TOM WALLGREN

Jackson, Mo.