understanding the reason a calculation occured in a joint dependent experiment












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i don't understand the multipication by 1/2 in the calculation. i'll provide description of the problem and what i did, in the calculation i do not understand.



there are 2 cages; in cage 1 there are one male and one female of some species. in cage 2 there are two females and two females of the same species. randomly, we choose a cage and take two species out of it.we define N - the number of cage the species were taken from. Z - number of male species taken.



i need to find the joint probability function of N and Z and to check if they are dependent or not.



the part i am having problem with:



so we can look at it like a bernoli trial, if cage one was chosen the only option is there is only one male(Z is hypergeometric as i understand) so, if we choose N=2 and Z=0, then the probability $P{n=2,Z=0}$ is $P{n=2,Z=0}=P{n=2|Z=0}P{n=2}=frac{binom20binom22}{binom42} * 1/2$. but i don't understand the 1/2: is it because i chose 1 cage out of 2 or is it something different? sorry for the "lame" question, i am in the beginning of my learning curve.



thank you very much! (p.s: in my opinion N and Z are dependent, since P{N=i}P{Z=j} are not going to be equal to P{N=i,Z=j}, in my opinion).










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    0












    $begingroup$


    i don't understand the multipication by 1/2 in the calculation. i'll provide description of the problem and what i did, in the calculation i do not understand.



    there are 2 cages; in cage 1 there are one male and one female of some species. in cage 2 there are two females and two females of the same species. randomly, we choose a cage and take two species out of it.we define N - the number of cage the species were taken from. Z - number of male species taken.



    i need to find the joint probability function of N and Z and to check if they are dependent or not.



    the part i am having problem with:



    so we can look at it like a bernoli trial, if cage one was chosen the only option is there is only one male(Z is hypergeometric as i understand) so, if we choose N=2 and Z=0, then the probability $P{n=2,Z=0}$ is $P{n=2,Z=0}=P{n=2|Z=0}P{n=2}=frac{binom20binom22}{binom42} * 1/2$. but i don't understand the 1/2: is it because i chose 1 cage out of 2 or is it something different? sorry for the "lame" question, i am in the beginning of my learning curve.



    thank you very much! (p.s: in my opinion N and Z are dependent, since P{N=i}P{Z=j} are not going to be equal to P{N=i,Z=j}, in my opinion).










    share|cite|improve this question









    $endgroup$















      0












      0








      0





      $begingroup$


      i don't understand the multipication by 1/2 in the calculation. i'll provide description of the problem and what i did, in the calculation i do not understand.



      there are 2 cages; in cage 1 there are one male and one female of some species. in cage 2 there are two females and two females of the same species. randomly, we choose a cage and take two species out of it.we define N - the number of cage the species were taken from. Z - number of male species taken.



      i need to find the joint probability function of N and Z and to check if they are dependent or not.



      the part i am having problem with:



      so we can look at it like a bernoli trial, if cage one was chosen the only option is there is only one male(Z is hypergeometric as i understand) so, if we choose N=2 and Z=0, then the probability $P{n=2,Z=0}$ is $P{n=2,Z=0}=P{n=2|Z=0}P{n=2}=frac{binom20binom22}{binom42} * 1/2$. but i don't understand the 1/2: is it because i chose 1 cage out of 2 or is it something different? sorry for the "lame" question, i am in the beginning of my learning curve.



      thank you very much! (p.s: in my opinion N and Z are dependent, since P{N=i}P{Z=j} are not going to be equal to P{N=i,Z=j}, in my opinion).










      share|cite|improve this question









      $endgroup$




      i don't understand the multipication by 1/2 in the calculation. i'll provide description of the problem and what i did, in the calculation i do not understand.



      there are 2 cages; in cage 1 there are one male and one female of some species. in cage 2 there are two females and two females of the same species. randomly, we choose a cage and take two species out of it.we define N - the number of cage the species were taken from. Z - number of male species taken.



      i need to find the joint probability function of N and Z and to check if they are dependent or not.



      the part i am having problem with:



      so we can look at it like a bernoli trial, if cage one was chosen the only option is there is only one male(Z is hypergeometric as i understand) so, if we choose N=2 and Z=0, then the probability $P{n=2,Z=0}$ is $P{n=2,Z=0}=P{n=2|Z=0}P{n=2}=frac{binom20binom22}{binom42} * 1/2$. but i don't understand the 1/2: is it because i chose 1 cage out of 2 or is it something different? sorry for the "lame" question, i am in the beginning of my learning curve.



      thank you very much! (p.s: in my opinion N and Z are dependent, since P{N=i}P{Z=j} are not going to be equal to P{N=i,Z=j}, in my opinion).







      probability probability-distributions






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      asked Jan 28 at 14:11









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