In this page is the percentage of different tornadoes and the fujita scale used to measure the intensity of tornadoes and their effects.
The percentage of weak tornadoes which occur is 88% but only causes around 4% of all tornado-related deaths. They generally last up to 10mins and are less than 175km/h in wind speed.
The percentage of strong tornadoes which occur is 11% and cause around 28% of all tornado-related deaths. They generally last around 20mins and are 175km/h to 340km/h in wind speed.
The percentage of violent tornadoes which occur is only 1% but cause around 68% of all tornado-related deaths. They generally last around an hour and are more than 340km/h in wind speed.
Please click on this link for the Fujita scale used to measure the strength of tornadoes: Fujita Scale