Define a probability distribution which satisfies independence conditions












0












$begingroup$


I have the following problem:



A certain hospital receives patients. 
Each patient could be healthy or sick.
There are 2 doctors: Gabby, and Tully.
Each doctor writes a report with his opinion.

There are 3 random variables: P, G, T with values {0,1} when 1 indicates sick and 0 indicates healthy.
P is the real status of the patients (Sick or healthy).
P[P=0] = P[P=1] = 0.5.

G, T indicates the doctors (Gabby, Tully) opinion.

Describe a distribution above P, G, T so that G, T will be independent,
but G, T are not independent given P.

The solution should be a table with all possible values of P, G, T
with the correct distribution for each combination.


My question is how to approach such a question?



I've tried making a table like this:



P | G | T | P(G|P) | P(T|P) | P(G,T|P) 
0 | 0 | 0 | 0.3 | 0.6 | 0.5
0 | 0 | 1 | 0.3 | 0.6 | 0.5
0 | 1 | 0 | 0.3 | 0.6 | 0.5
0 | 1 | 1 | 0.3 | 0.6 | 0.5
1 | 0 | 0 | 0.3 | 0.6 | 0.5
1 | 0 | 1 | 0.3 | 0.6 | 0.5
1 | 1 | 0 | 0.3 | 0.6 | 0.5
1 | 1 | 1 | 0.3 | 0.6 | 0.5


And now I have P(G,T|P) != P(G|P) * P(T|P) as needed,



(Because of P(G|P) = 0.3, P(T|P) = 0.6, P(G|P) * P(T|P) = 0.5)



But I'm not sure how to calculate P(G), P(T), and check their independence.



Will be glad to know to approach this problem. Thanks.



Edit: This is my current solution:



Distribution



Independence










share|cite|improve this question











$endgroup$












  • $begingroup$
    In the table you correctly specify all possible outcomes $(0,0,0), (0,0,1), ldots$ but you are missing the probability of each outcome! That is, what is the probability $mathbb{P}(P = 0, G=0, T=0)$? Also I am not sure about the meaning of $mathbb{P}(G|P)$. Maybe you meant $mathbb{P}(G=x|P=y)$ for all $x,y in {0,1}$?
    $endgroup$
    – Michh
    Dec 29 '18 at 18:56










  • $begingroup$
    First, for your question - it means P(G=x | P=y) for the values given in the table. For example in the first row, P(G=0 | P=0). Second, I've tried to add the probabilities such P(P=0,G=0,T=0) but I still dont understand how to satisfies all the constraints
    $endgroup$
    – JohnSnowTheDeveloper
    Dec 29 '18 at 19:17


















0












$begingroup$


I have the following problem:



A certain hospital receives patients. 
Each patient could be healthy or sick.
There are 2 doctors: Gabby, and Tully.
Each doctor writes a report with his opinion.

There are 3 random variables: P, G, T with values {0,1} when 1 indicates sick and 0 indicates healthy.
P is the real status of the patients (Sick or healthy).
P[P=0] = P[P=1] = 0.5.

G, T indicates the doctors (Gabby, Tully) opinion.

Describe a distribution above P, G, T so that G, T will be independent,
but G, T are not independent given P.

The solution should be a table with all possible values of P, G, T
with the correct distribution for each combination.


My question is how to approach such a question?



I've tried making a table like this:



P | G | T | P(G|P) | P(T|P) | P(G,T|P) 
0 | 0 | 0 | 0.3 | 0.6 | 0.5
0 | 0 | 1 | 0.3 | 0.6 | 0.5
0 | 1 | 0 | 0.3 | 0.6 | 0.5
0 | 1 | 1 | 0.3 | 0.6 | 0.5
1 | 0 | 0 | 0.3 | 0.6 | 0.5
1 | 0 | 1 | 0.3 | 0.6 | 0.5
1 | 1 | 0 | 0.3 | 0.6 | 0.5
1 | 1 | 1 | 0.3 | 0.6 | 0.5


And now I have P(G,T|P) != P(G|P) * P(T|P) as needed,



(Because of P(G|P) = 0.3, P(T|P) = 0.6, P(G|P) * P(T|P) = 0.5)



But I'm not sure how to calculate P(G), P(T), and check their independence.



Will be glad to know to approach this problem. Thanks.



Edit: This is my current solution:



Distribution



Independence










share|cite|improve this question











$endgroup$












  • $begingroup$
    In the table you correctly specify all possible outcomes $(0,0,0), (0,0,1), ldots$ but you are missing the probability of each outcome! That is, what is the probability $mathbb{P}(P = 0, G=0, T=0)$? Also I am not sure about the meaning of $mathbb{P}(G|P)$. Maybe you meant $mathbb{P}(G=x|P=y)$ for all $x,y in {0,1}$?
    $endgroup$
    – Michh
    Dec 29 '18 at 18:56










  • $begingroup$
    First, for your question - it means P(G=x | P=y) for the values given in the table. For example in the first row, P(G=0 | P=0). Second, I've tried to add the probabilities such P(P=0,G=0,T=0) but I still dont understand how to satisfies all the constraints
    $endgroup$
    – JohnSnowTheDeveloper
    Dec 29 '18 at 19:17
















0












0








0


1



$begingroup$


I have the following problem:



A certain hospital receives patients. 
Each patient could be healthy or sick.
There are 2 doctors: Gabby, and Tully.
Each doctor writes a report with his opinion.

There are 3 random variables: P, G, T with values {0,1} when 1 indicates sick and 0 indicates healthy.
P is the real status of the patients (Sick or healthy).
P[P=0] = P[P=1] = 0.5.

G, T indicates the doctors (Gabby, Tully) opinion.

Describe a distribution above P, G, T so that G, T will be independent,
but G, T are not independent given P.

The solution should be a table with all possible values of P, G, T
with the correct distribution for each combination.


My question is how to approach such a question?



I've tried making a table like this:



P | G | T | P(G|P) | P(T|P) | P(G,T|P) 
0 | 0 | 0 | 0.3 | 0.6 | 0.5
0 | 0 | 1 | 0.3 | 0.6 | 0.5
0 | 1 | 0 | 0.3 | 0.6 | 0.5
0 | 1 | 1 | 0.3 | 0.6 | 0.5
1 | 0 | 0 | 0.3 | 0.6 | 0.5
1 | 0 | 1 | 0.3 | 0.6 | 0.5
1 | 1 | 0 | 0.3 | 0.6 | 0.5
1 | 1 | 1 | 0.3 | 0.6 | 0.5


And now I have P(G,T|P) != P(G|P) * P(T|P) as needed,



(Because of P(G|P) = 0.3, P(T|P) = 0.6, P(G|P) * P(T|P) = 0.5)



But I'm not sure how to calculate P(G), P(T), and check their independence.



Will be glad to know to approach this problem. Thanks.



Edit: This is my current solution:



Distribution



Independence










share|cite|improve this question











$endgroup$




I have the following problem:



A certain hospital receives patients. 
Each patient could be healthy or sick.
There are 2 doctors: Gabby, and Tully.
Each doctor writes a report with his opinion.

There are 3 random variables: P, G, T with values {0,1} when 1 indicates sick and 0 indicates healthy.
P is the real status of the patients (Sick or healthy).
P[P=0] = P[P=1] = 0.5.

G, T indicates the doctors (Gabby, Tully) opinion.

Describe a distribution above P, G, T so that G, T will be independent,
but G, T are not independent given P.

The solution should be a table with all possible values of P, G, T
with the correct distribution for each combination.


My question is how to approach such a question?



I've tried making a table like this:



P | G | T | P(G|P) | P(T|P) | P(G,T|P) 
0 | 0 | 0 | 0.3 | 0.6 | 0.5
0 | 0 | 1 | 0.3 | 0.6 | 0.5
0 | 1 | 0 | 0.3 | 0.6 | 0.5
0 | 1 | 1 | 0.3 | 0.6 | 0.5
1 | 0 | 0 | 0.3 | 0.6 | 0.5
1 | 0 | 1 | 0.3 | 0.6 | 0.5
1 | 1 | 0 | 0.3 | 0.6 | 0.5
1 | 1 | 1 | 0.3 | 0.6 | 0.5


And now I have P(G,T|P) != P(G|P) * P(T|P) as needed,



(Because of P(G|P) = 0.3, P(T|P) = 0.6, P(G|P) * P(T|P) = 0.5)



But I'm not sure how to calculate P(G), P(T), and check their independence.



Will be glad to know to approach this problem. Thanks.



Edit: This is my current solution:



Distribution



Independence







probability probability-distributions random-variables bayes-theorem






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Jan 8 at 10:00









TZakrevskiy

20.2k12353




20.2k12353










asked Dec 29 '18 at 16:44









JohnSnowTheDeveloperJohnSnowTheDeveloper

656




656












  • $begingroup$
    In the table you correctly specify all possible outcomes $(0,0,0), (0,0,1), ldots$ but you are missing the probability of each outcome! That is, what is the probability $mathbb{P}(P = 0, G=0, T=0)$? Also I am not sure about the meaning of $mathbb{P}(G|P)$. Maybe you meant $mathbb{P}(G=x|P=y)$ for all $x,y in {0,1}$?
    $endgroup$
    – Michh
    Dec 29 '18 at 18:56










  • $begingroup$
    First, for your question - it means P(G=x | P=y) for the values given in the table. For example in the first row, P(G=0 | P=0). Second, I've tried to add the probabilities such P(P=0,G=0,T=0) but I still dont understand how to satisfies all the constraints
    $endgroup$
    – JohnSnowTheDeveloper
    Dec 29 '18 at 19:17




















  • $begingroup$
    In the table you correctly specify all possible outcomes $(0,0,0), (0,0,1), ldots$ but you are missing the probability of each outcome! That is, what is the probability $mathbb{P}(P = 0, G=0, T=0)$? Also I am not sure about the meaning of $mathbb{P}(G|P)$. Maybe you meant $mathbb{P}(G=x|P=y)$ for all $x,y in {0,1}$?
    $endgroup$
    – Michh
    Dec 29 '18 at 18:56










  • $begingroup$
    First, for your question - it means P(G=x | P=y) for the values given in the table. For example in the first row, P(G=0 | P=0). Second, I've tried to add the probabilities such P(P=0,G=0,T=0) but I still dont understand how to satisfies all the constraints
    $endgroup$
    – JohnSnowTheDeveloper
    Dec 29 '18 at 19:17


















$begingroup$
In the table you correctly specify all possible outcomes $(0,0,0), (0,0,1), ldots$ but you are missing the probability of each outcome! That is, what is the probability $mathbb{P}(P = 0, G=0, T=0)$? Also I am not sure about the meaning of $mathbb{P}(G|P)$. Maybe you meant $mathbb{P}(G=x|P=y)$ for all $x,y in {0,1}$?
$endgroup$
– Michh
Dec 29 '18 at 18:56




$begingroup$
In the table you correctly specify all possible outcomes $(0,0,0), (0,0,1), ldots$ but you are missing the probability of each outcome! That is, what is the probability $mathbb{P}(P = 0, G=0, T=0)$? Also I am not sure about the meaning of $mathbb{P}(G|P)$. Maybe you meant $mathbb{P}(G=x|P=y)$ for all $x,y in {0,1}$?
$endgroup$
– Michh
Dec 29 '18 at 18:56












$begingroup$
First, for your question - it means P(G=x | P=y) for the values given in the table. For example in the first row, P(G=0 | P=0). Second, I've tried to add the probabilities such P(P=0,G=0,T=0) but I still dont understand how to satisfies all the constraints
$endgroup$
– JohnSnowTheDeveloper
Dec 29 '18 at 19:17






$begingroup$
First, for your question - it means P(G=x | P=y) for the values given in the table. For example in the first row, P(G=0 | P=0). Second, I've tried to add the probabilities such P(P=0,G=0,T=0) but I still dont understand how to satisfies all the constraints
$endgroup$
– JohnSnowTheDeveloper
Dec 29 '18 at 19:17












1 Answer
1






active

oldest

votes


















3












$begingroup$

As Misch said, in each row of the table you should write $mathbb{P}( P = y, G= x, T = z )$, where $x,y,z$ equal $0$ or $1$. Then:




  • All eight probabilities should sum to $1$ - they represent all possibilities.

  • You calculate $mathbb{P}(P = y )$ by taking the sum over the four rows where $P = y$. This sum should equal $0.5$ for both $y=0$ and $y=1$.

  • You calculate $mathbb{P}(G = x )$ by taking the sum over the four rows where $G = x$, and find $mathbb{P}(T = z )$ in the same way.

  • You calculate $mathbb{P}(G = x, T = z )$ by taking the sum over the two rows where both $G=x$ and $T=z$.


To show that $G$ and $T$ are independent, you must show that $mathbb{P}(G = x ) mathbb{P}(T = z ) = mathbb{P}(G = x, T =z )$ for all four possible values of $(x,z)$. To show that $G$ and $T$ are not conditionally independent given $P$, it is enough to find one set of values for $(x,y,z)$ where $mathbb{P}( G = x | P = y) mathbb{P}( T = z | P = y) neq mathbb{P}( G = x, T = z | P = y)$.



As there are infinitely many solutions, it may help if you add your own constraints. For instance, you may set $mathbb{P}(G = x ) = 0.5$ for $x=0,1$, and $mathbb{P}(T = z ) = 0.5$ for $ z=0,1$.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Thanks, I've edited my question to include the answer, I hope I understood you well (Feel free to fix me if I'm wrong)
    $endgroup$
    – JohnSnowTheDeveloper
    Dec 30 '18 at 19:13











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1 Answer
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1 Answer
1






active

oldest

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active

oldest

votes






active

oldest

votes









3












$begingroup$

As Misch said, in each row of the table you should write $mathbb{P}( P = y, G= x, T = z )$, where $x,y,z$ equal $0$ or $1$. Then:




  • All eight probabilities should sum to $1$ - they represent all possibilities.

  • You calculate $mathbb{P}(P = y )$ by taking the sum over the four rows where $P = y$. This sum should equal $0.5$ for both $y=0$ and $y=1$.

  • You calculate $mathbb{P}(G = x )$ by taking the sum over the four rows where $G = x$, and find $mathbb{P}(T = z )$ in the same way.

  • You calculate $mathbb{P}(G = x, T = z )$ by taking the sum over the two rows where both $G=x$ and $T=z$.


To show that $G$ and $T$ are independent, you must show that $mathbb{P}(G = x ) mathbb{P}(T = z ) = mathbb{P}(G = x, T =z )$ for all four possible values of $(x,z)$. To show that $G$ and $T$ are not conditionally independent given $P$, it is enough to find one set of values for $(x,y,z)$ where $mathbb{P}( G = x | P = y) mathbb{P}( T = z | P = y) neq mathbb{P}( G = x, T = z | P = y)$.



As there are infinitely many solutions, it may help if you add your own constraints. For instance, you may set $mathbb{P}(G = x ) = 0.5$ for $x=0,1$, and $mathbb{P}(T = z ) = 0.5$ for $ z=0,1$.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Thanks, I've edited my question to include the answer, I hope I understood you well (Feel free to fix me if I'm wrong)
    $endgroup$
    – JohnSnowTheDeveloper
    Dec 30 '18 at 19:13
















3












$begingroup$

As Misch said, in each row of the table you should write $mathbb{P}( P = y, G= x, T = z )$, where $x,y,z$ equal $0$ or $1$. Then:




  • All eight probabilities should sum to $1$ - they represent all possibilities.

  • You calculate $mathbb{P}(P = y )$ by taking the sum over the four rows where $P = y$. This sum should equal $0.5$ for both $y=0$ and $y=1$.

  • You calculate $mathbb{P}(G = x )$ by taking the sum over the four rows where $G = x$, and find $mathbb{P}(T = z )$ in the same way.

  • You calculate $mathbb{P}(G = x, T = z )$ by taking the sum over the two rows where both $G=x$ and $T=z$.


To show that $G$ and $T$ are independent, you must show that $mathbb{P}(G = x ) mathbb{P}(T = z ) = mathbb{P}(G = x, T =z )$ for all four possible values of $(x,z)$. To show that $G$ and $T$ are not conditionally independent given $P$, it is enough to find one set of values for $(x,y,z)$ where $mathbb{P}( G = x | P = y) mathbb{P}( T = z | P = y) neq mathbb{P}( G = x, T = z | P = y)$.



As there are infinitely many solutions, it may help if you add your own constraints. For instance, you may set $mathbb{P}(G = x ) = 0.5$ for $x=0,1$, and $mathbb{P}(T = z ) = 0.5$ for $ z=0,1$.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Thanks, I've edited my question to include the answer, I hope I understood you well (Feel free to fix me if I'm wrong)
    $endgroup$
    – JohnSnowTheDeveloper
    Dec 30 '18 at 19:13














3












3








3





$begingroup$

As Misch said, in each row of the table you should write $mathbb{P}( P = y, G= x, T = z )$, where $x,y,z$ equal $0$ or $1$. Then:




  • All eight probabilities should sum to $1$ - they represent all possibilities.

  • You calculate $mathbb{P}(P = y )$ by taking the sum over the four rows where $P = y$. This sum should equal $0.5$ for both $y=0$ and $y=1$.

  • You calculate $mathbb{P}(G = x )$ by taking the sum over the four rows where $G = x$, and find $mathbb{P}(T = z )$ in the same way.

  • You calculate $mathbb{P}(G = x, T = z )$ by taking the sum over the two rows where both $G=x$ and $T=z$.


To show that $G$ and $T$ are independent, you must show that $mathbb{P}(G = x ) mathbb{P}(T = z ) = mathbb{P}(G = x, T =z )$ for all four possible values of $(x,z)$. To show that $G$ and $T$ are not conditionally independent given $P$, it is enough to find one set of values for $(x,y,z)$ where $mathbb{P}( G = x | P = y) mathbb{P}( T = z | P = y) neq mathbb{P}( G = x, T = z | P = y)$.



As there are infinitely many solutions, it may help if you add your own constraints. For instance, you may set $mathbb{P}(G = x ) = 0.5$ for $x=0,1$, and $mathbb{P}(T = z ) = 0.5$ for $ z=0,1$.






share|cite|improve this answer











$endgroup$



As Misch said, in each row of the table you should write $mathbb{P}( P = y, G= x, T = z )$, where $x,y,z$ equal $0$ or $1$. Then:




  • All eight probabilities should sum to $1$ - they represent all possibilities.

  • You calculate $mathbb{P}(P = y )$ by taking the sum over the four rows where $P = y$. This sum should equal $0.5$ for both $y=0$ and $y=1$.

  • You calculate $mathbb{P}(G = x )$ by taking the sum over the four rows where $G = x$, and find $mathbb{P}(T = z )$ in the same way.

  • You calculate $mathbb{P}(G = x, T = z )$ by taking the sum over the two rows where both $G=x$ and $T=z$.


To show that $G$ and $T$ are independent, you must show that $mathbb{P}(G = x ) mathbb{P}(T = z ) = mathbb{P}(G = x, T =z )$ for all four possible values of $(x,z)$. To show that $G$ and $T$ are not conditionally independent given $P$, it is enough to find one set of values for $(x,y,z)$ where $mathbb{P}( G = x | P = y) mathbb{P}( T = z | P = y) neq mathbb{P}( G = x, T = z | P = y)$.



As there are infinitely many solutions, it may help if you add your own constraints. For instance, you may set $mathbb{P}(G = x ) = 0.5$ for $x=0,1$, and $mathbb{P}(T = z ) = 0.5$ for $ z=0,1$.







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited Dec 30 '18 at 14:36

























answered Dec 29 '18 at 21:13









JAskgaardJAskgaard

1367




1367












  • $begingroup$
    Thanks, I've edited my question to include the answer, I hope I understood you well (Feel free to fix me if I'm wrong)
    $endgroup$
    – JohnSnowTheDeveloper
    Dec 30 '18 at 19:13


















  • $begingroup$
    Thanks, I've edited my question to include the answer, I hope I understood you well (Feel free to fix me if I'm wrong)
    $endgroup$
    – JohnSnowTheDeveloper
    Dec 30 '18 at 19:13
















$begingroup$
Thanks, I've edited my question to include the answer, I hope I understood you well (Feel free to fix me if I'm wrong)
$endgroup$
– JohnSnowTheDeveloper
Dec 30 '18 at 19:13




$begingroup$
Thanks, I've edited my question to include the answer, I hope I understood you well (Feel free to fix me if I'm wrong)
$endgroup$
– JohnSnowTheDeveloper
Dec 30 '18 at 19:13


















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