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Re: Probability..

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Thanks again to Ahmed Nisar for clarifying terms of probability: 
>The difference in probability of exceedence and probability of occurrence can 
>be understood this way.. 
>Suppose you throw two dice, the probability of getting an exact seven, no 
>no less is the probability of occurrence of seven.  the probability of  
>exceedence will be the probability of getting anything greater than an exact 
It occurred to me after asking the question that probability of occurrence 
refers to the probability of a single event occurring whereas probability of 
exceedance refers to any event "equal to or exceeding".  Thus when ASCE 7 
refers to "annual probability" for wind speed, it is referring to probability 
of exceedance; similarly, for floods we refer to expected return period which 
relates to exceedance probability.  
Thus looking back at the original message which prompted my questions, Frank 
McClure questioned the following statement: "probabilistic methods could help 
inform the decision, with consideration of the likelihood of a significant 
seismic event occurring in the time before remedial work, the risk of collapse 
due to that event relative to the risk of collapse of some other "typical" 
building during a similar exposure period, etc.".  
His response was "We start down a very slippery slope if we starting telling 
our clients or the public that the probability of occurrence can be given in 
terms of a certain number of years for earthquakes of a certain magnitude or 
ground motion 
strength.  Earthquake ground motion strength parameters are usually, for 
building code purposes or earthquake risk assessments, expressed in terms of 
While I concur that probability of occurrence of a single event is different 
than probability of exceedance based on the magnitude of an event, I don't see 
how that prevents us from discussing seismic events based on the probability 
of a certain magnitude event during a given number of years (if that was the 
point).  Nevertheless, I do think that expressing the probability in terms of 
the probability of exceedance over a given design life is more meaningful than 
stating the "mean" return period, as a return period longer than the design 
life seems to give a false sense of security - e.g., a 100-year flood design 
for a 50-year design life sounds conservative, but a 40% probability of 
exceedance in 50-years sounds risky.  It would be beneficial if all such 
events - earthquake, wind, flood, etc - were expressed using consistent terms.