# Sensitivity Factors from FORM method¶

(1)¶

The sensitivity factors indicate the importance on the Hasofer-Lind reliability index (refer to ) of the value of the parameters used to define the distribution of the random vector .

Here, the event considered is explicited directly from the limit state function : this is the classical structural reliability formulation. However, if the event is a threshold exceedance, it is useful to explicit the variable of interest , evaluated from the model . In that case, the event considered, associated to the threshold has the formulation: and the limit state function is : . is the threshold exceedance probability, defined as: . Thus, the FORM sensitivity factors offer a way to rank the importance of the parameters of the input components with respect to the threshold exceedance by the quantity of interest . They can be seen as a specific sensitivity analysis technique dedicated to the quantity Z around a particular threshold rather than to its variance.