# Markov chain duration [closed]

What is the formula to find average duration of state s in a Markov chain given a transition matrix?

I tried to recall the concept but could not find any references.

## closed as off-topic by Did, d80d2729a352b1366139fc119d3345, José Carlos Santos, mrtaurho, Riccardo.AlestraJan 25 at 15:02

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I understand that we are talking about a discrete time process. If the probability of staying in state $$s$$ from one period to another is $$P(X_n=s|X_{n-1}=s)=q,\quad \forall n\ge1,$$ then the number $$N$$ of consecutive repetitions of the state $$s$$ is a random variable with geometric distribution, that is $$N\sim \mathcal G(1-q),$$ since every transition can be seen as a dicotomic experiment where the "fail" consists in staying at state $$s$$ (with probability $$1-q$$) and "success" is changing to any other state (with probability $$q$$).
In fact, there are several alternative definitions of a geometric r.v., but here I consider it as the number of "fail" results before the first "success". In this case, we have $$E(N)=\frac q{1-q}$$.
Since the actual number of successive $$s$$ states would be $$N+1$$, if we consider the first time we reach that state, the expected number of $$s$$ states once the chain reaches it would be $$E(N+1)=1+\frac q{1-q}=\frac1{1-q},$$ counting the current time, or $$E(N)=\frac q{1-q},$$ if we count from next time on.
• Take $q=7/8$ and you'll get the result. – Alejandro Nasif Salum Jan 25 at 6:52