Markov Property

Discussion in 'CT4' started by Harashima Senju, Sep 19, 2017.

  1. Harashima Senju

    Harashima Senju Ton up Member

    Does an IID sequence imply that it statisfies the Markov property?
     
  2. John Potter

    John Potter ActEd Tutor Staff Member

    Yes, in fact, this is a good example of processes that have the Markov property but DO NOT have independent increments.

    A process with independent increments satisfies the Markov property. However, not the other way round.

    Consider a IID sequence Zn = 1 or -1 on the toss of a coin (50/50)
    It's trivially Markov because P[Z2=1|Z1] = P[Z2=1|F1]
    (each coin toss is independent of the last, so knowing the history won't change our predictions of the future)

    Now, what is P[Z2 - Z1] = 2?
    0.25, since we must have Z1 = -1 and Z2 = 1
    But what if we know that Z1 - Z0 = 2. Now, what is P[Z2 - Z1] = 2?
    0 - it's impossible because we must have Z0 = -1 and Z1 = 1.
    So, the increment Z1 - Z0 is NOT independent of Z2 - Z1
    I'd arm myself with this example just in case they ask for one in the CT4 exam (though they never do!)

    Good luck!
    John
     
    Harashima Senju likes this.

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