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					   Biostatistics

             NAN-YING YU
                 3/27/2008
                 4/03/2008

Chapter 7 Theoretical Probability
          Distributions
7.1 Probability Distributions




                   =
7.2 The binomial distribution

It involves three assumptions
   1. There are a fixed number of trials n, each
      of which results in one of two mutually
      exclusive outcomes.
   2. The outcomes of the n trials are
      independent
   3. The probability of success p is constant for
      each trial.
General expression for the probability
distribution of a binomial random variable X

                      n!
    P( X  x)               p x (1  p ) n  x
                  x!(n  x)!
                n
                
                 x         p x (1  p ) n  x
                 

     Mean number  np
     Standard deviation  np(1  p)
General expression for a Poisson
distribution


                    e   x
        P( X  x) 
                       x!

          np
=0
=1
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