I grew up in the generalized area known as Tornado Alley. Every Spring and Summer the sirens would go off and we would grab the transistor radio and head to the basement. While a basement won't guarantee 100% that a person will escape the wrath of a tornado, your chances of survival are much greater. In all the years I lived in Tornado land, I can only think of two houses I lived in that did not have a basement and I didn't live in either for very long.
It is a known fact that riding out a tornado below the ground floor -- even a direct hit -- saves a tremendous amount of lives. And yet, few communities in Tornado Alley require basements for newly built homes and public buildings. One of the two schools in Moore, Oklahoma -- Plaza Towers Elementary School -- that suffered a direct hit by a tornado on Monday did not have a basement and several children were killed. Many of the survivors interviewed survived because they lived in homes that had basements (or storm cellars) and they were able to get down in them before the twister hit and destroyed their homes.
Since we know that basements save lives in tornadoes, why aren't they mandated in all new construction? The answer is as old as they hills: It costs more. Builders don't want the added costs and neither do many homeowners. So, homes are built on slabs in Tornado Alley and the people who live in them often die needlessly when a tornado descends on their neighborhood.