You need to consider the relative sizes of the initial populations of the
different species. For example if there are 100 red fish for every blue
fish, you wouldn't expect to find many blue fish in a sample of 6. So you
can't assume necessarily that different species have a random chance of
joining the community--it depends on prior probabilities.
Sara
-----Original Message-----
From: Coomaren Pillay Vencatasawmy <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Wednesday, August 18, 1999 8:40 AM
Subject: Help with a probability question
>Dear all
>
>I got this request from a friend:
>
>In theory, the more fish there is on a reef, the more species are
>expected
>to exist by chance (if fish of different species have a random chance of
>joining the community). My own hypothesis is different so I am trying to
>disprove the hypothesis that different fish species appear at random in
>a
>fish community.
>
>So he took 150 samples, each containing up to 12 fishes.
>
>For example he has (out of a maximum of 7 species)
>
>sample 1 6 fish 2 species
>sample 2 4 fish 1 species
>sample 3 8 fish 5 species
>
>He needs to determine the expected number of species in each sample or
>the probability of observing sample 1.
>
>According to me (and him) he is testing the hypothesis prob(getting
>species i)=1/7. So in principle in a sample of 6, he should get 6
>species.
>
>This seems intuitively wrong.
>
>Can anyone help please?
>
>Coomaren
>
>
>........................................................................
>........................................................
>Dr Coomaren P Vencatasawmy
>Senior Research Fellow
>Spatial Modelling Centre (SMC)
>P.O. Box 839
>S-981 28
>Kiruna
>Sweden
>[log in to unmask]
>Telephone: 46 980 67013
>Fax : 46 980 67190
>"Perfection is achieved, not when there is nothing more to add, but when
>there is nothing left to take away." - Antoine de Saint Exupery
>........................................................................
>........................................................
>
>
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