Rice's formula
In probability theory, Rice's formula counts the average number of times an ergodic stationary process X(t) per unit time crosses a fixed level u.[1] Adler and Taylor describe the result as "one of the most important results in the applications of smooth stochastic processes."[2]
Contents
History
The formula was published by Stephen O. Rice in 1944,[3] having previously been discussed in his 1936 note entitled "Singing Transmission Lines."[4][5]
Formula
Write Du for the number of times the ergodic stationary stochastic process X(t) takes the value u in a unit of time (i.e. t ∈ [0,1]). Then Rice's formula states that
where p(x,x') is the joint probability density of the X(t) and its mean-square derivative X'(t).[6]
If the process X(t) is a Gaussian process and u = 0 then the formula simplifies significantly to give[6][7]
where ρ'' is the second derivative of the normalised autocorrelation of X(t) at 0.
Uses
Rice's formula can be used to approximate an excursion probability[8]
as for large values of u the probability that there is a level crossing is approximately the probability of reaching that level.
References
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