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SIS Epidemic Modelling

  1. May 9, 2013 #1
    For the probability of the number of infective's increasing in one time step, I found the answer is:
    where β is the contact rate, ∆t is the time step, i is number of infectives,N is total number of susceptible's and infective's

    I can't quite see where this is coming from. β is the contact rate, so it makes sense to multiply by i since for more i, more chances of encounters. Then multiplying by N-i makes sense since more susceptible also means more chance of encounters.

    But then why the division by N? I think my problem may be in understanding what the "contact rate" actually means
  2. jcsd
  3. May 10, 2013 #2
    I think it actually has to do with understanding what a probability is. :tongue:

    What is a probability? What does it tell you?

    Answering those questions should tell you the answer to your question.
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