5
$\begingroup$

I would like to understand better the derivation of the inter-arrival time for a nonhomogeneous Poisson process. Can any one supply a link to a nice clear derivation? I'm only interested in the time to the first event so the survival function would be great but general case is fine. The only references I can find essentially state the result without a derivation.

Let $T_1, T_2, \dots$ denote the interarrival times of events of a nonhomogeneous Poisson process having intensity function $\lambda(t)$.

  1. Are the $T_i$ independent?
  2. Are the $T_i$ identically distributed?
  3. Find the distribution of $T_1$.
$\endgroup$

1 Answer 1

2
$\begingroup$

In general, the times $T_i$ are not independent and not identically distributed. So questions 1 and 2 are answered in the negative. It is not too difficult to find examples in which they are clearly dependent and/or clearly differently distributed. I leave such a demonstration for someone else but shall however try to derive the distribution of $T_1$.

First recall that if $\lambda(t)=\lambda$ is fixed (so in case of the usual homogeneous Poisson process), the probability that there is no point in an interval of length $s$ is $e^{-\lambda s}$. This is because we know that the number of points $N_s$ in an interval of length $s$ is Poisson distributed with parameter $\lambda s$ (constant rate $\times$ interval length): $$ \text{Prob}[N_s = n] = e^{-\lambda s} \frac{(\lambda s)^n}{n!} $$ so $$ \text{Prob}[N_s = 0] = e^{-\lambda s} $$

Secondly, consider the probability that no point occurs in the interval $[0,s]$ for the nonhomogeneous process. To obtain this probability, we are going to cut the interval in equal parts of length $\Delta$ and then let $\Delta\to 0$. Every part then becomes very (infinitesimally) small so that we can assume the rate $\lambda(t)$ is constant in that small part. If the rate is constant there (say equal to $\lambda$), the number of points in that part is Poisson distributed with parameter $\lambda \Delta$. So we have: \begin{align*} \text{Prob}\big[\text{no point in }[0,s]\big] & = \lim_{\Delta\to 0}\text{Prob}\big[\text{no point in }[0,\Delta], \ldots, \text{no point in }[s-\Delta,s]\big]\\ & = \lim_{\Delta\to 0}\prod_{i=0}^{s/\Delta-1}\text{Prob}\big[\text{no point in }[i\Delta,(i+1)\Delta]\big]\\ & = \lim_{\Delta\to 0}\prod_{i=0}^{s/\Delta-1} e^{-\lambda(i\Delta)\Delta}\\ & = \lim_{\Delta\to 0} \exp \Big[ - \sum_{i=0}^{s/\Delta-1} \lambda(i\Delta)\Delta) \Big]\\ & = \exp \Big[ - \lim_{\Delta\to 0} \sum_{i=0}^{s/\Delta-1} \lambda(i\Delta)\Delta \Big] = \exp \Big[ - \int_0^s \lambda(t) \text{d}t \Big] = e^{-\Lambda(0,s)} \end{align*} because the last limit is nothing but a Riemann sum that is the area under the graph of $\lambda(t)$ from $t=0$ to $t=s$. For convenience, let us call $\Lambda(t_1,t_2) =\int_{t_1}^{t_2} \lambda(t)\text{d}t$.

Finally, consider the probability density $f(t)$ function of $T_1$: \begin{align*} f(t)\text{d}t & = \text{Prob}[ t \leqslant T_1 < t+\text{d}t ]\\ & = \text{Prob}\big[\text{no point in }[0,t]\big] \cdot \text{Prob}\big[\text{1 point in }[t,t+\text{d}t[\big]\\ & = e^{-\Lambda(0,t)} \cdot e^{-\lambda(t)\text{d}t}\lambda(t)\text{d}t = e^{-\Lambda(0,t)} \lambda(t)\text{d}t \end{align*} so the density of $T_1$ is $$ f(t) = e^{-\Lambda(0,t)} \lambda(t) $$ and the distribution function $F(t)=\text{Prob}[T\leqslant t]$ follows by integration of $f(t)$ as $$ F(t) = 1- e^{-\Lambda(0,t)} $$

$\endgroup$
1
  • $\begingroup$ Thanks a lot! This explanation really helped out my problem $\endgroup$
    – GorillaInR
    Feb 13, 2018 at 0:23

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.