Compute the probability $P(X ≤ Y|min{X,Y}=x)$
$begingroup$
Let $X$ and $Y$ be two independent exponential random variables with parameters $lambda$ and $mu$, respectively. Compute the probability $P(X ≤ Y|min{X,Y}=x)$.
Here is what I did:
If $Y=min{X,Y}$, then $Ylt X$, $P(X ≤ Y|min{X,Y}=x)=0$.
If $X=min{X,Y}$, then $Xlt Y$,
$$
begin{aligned}
P(X ≤ Y|min{X,Y}=x)&=P(X ≤ Y|X=x)\
&=frac{P(Ygt x)}{f_X(x)}\
&=frac{1-P(Ylt x)}{f_X(x)}\
&={1-int_{0}^xmu e^{-mu y}dyover lambda e^{-lambda x}}
end{aligned}
$$
Am I right?
probability
$endgroup$
add a comment |
$begingroup$
Let $X$ and $Y$ be two independent exponential random variables with parameters $lambda$ and $mu$, respectively. Compute the probability $P(X ≤ Y|min{X,Y}=x)$.
Here is what I did:
If $Y=min{X,Y}$, then $Ylt X$, $P(X ≤ Y|min{X,Y}=x)=0$.
If $X=min{X,Y}$, then $Xlt Y$,
$$
begin{aligned}
P(X ≤ Y|min{X,Y}=x)&=P(X ≤ Y|X=x)\
&=frac{P(Ygt x)}{f_X(x)}\
&=frac{1-P(Ylt x)}{f_X(x)}\
&={1-int_{0}^xmu e^{-mu y}dyover lambda e^{-lambda x}}
end{aligned}
$$
Am I right?
probability
$endgroup$
$begingroup$
No. You should answer in a single statement, not a case by case basis. Secondly, your proof is wrong because $P(X = x)$ is $0$ since $X$ is a continuous random variable, so you're dividing by zero
$endgroup$
– Xiaomi
Oct 1 '18 at 1:45
$begingroup$
@Xiaomi I improved my answer. Is that correct now? And how to answer in a single statement?
$endgroup$
– Yibei He
Oct 1 '18 at 1:52
$begingroup$
@YibeiHe, still no. $f_X(x) ne P(X=x)$
$endgroup$
– Zamarion
Oct 1 '18 at 1:54
add a comment |
$begingroup$
Let $X$ and $Y$ be two independent exponential random variables with parameters $lambda$ and $mu$, respectively. Compute the probability $P(X ≤ Y|min{X,Y}=x)$.
Here is what I did:
If $Y=min{X,Y}$, then $Ylt X$, $P(X ≤ Y|min{X,Y}=x)=0$.
If $X=min{X,Y}$, then $Xlt Y$,
$$
begin{aligned}
P(X ≤ Y|min{X,Y}=x)&=P(X ≤ Y|X=x)\
&=frac{P(Ygt x)}{f_X(x)}\
&=frac{1-P(Ylt x)}{f_X(x)}\
&={1-int_{0}^xmu e^{-mu y}dyover lambda e^{-lambda x}}
end{aligned}
$$
Am I right?
probability
$endgroup$
Let $X$ and $Y$ be two independent exponential random variables with parameters $lambda$ and $mu$, respectively. Compute the probability $P(X ≤ Y|min{X,Y}=x)$.
Here is what I did:
If $Y=min{X,Y}$, then $Ylt X$, $P(X ≤ Y|min{X,Y}=x)=0$.
If $X=min{X,Y}$, then $Xlt Y$,
$$
begin{aligned}
P(X ≤ Y|min{X,Y}=x)&=P(X ≤ Y|X=x)\
&=frac{P(Ygt x)}{f_X(x)}\
&=frac{1-P(Ylt x)}{f_X(x)}\
&={1-int_{0}^xmu e^{-mu y}dyover lambda e^{-lambda x}}
end{aligned}
$$
Am I right?
probability
probability
edited Oct 1 '18 at 1:51
Yibei He
asked Oct 1 '18 at 1:29
Yibei HeYibei He
3139
3139
$begingroup$
No. You should answer in a single statement, not a case by case basis. Secondly, your proof is wrong because $P(X = x)$ is $0$ since $X$ is a continuous random variable, so you're dividing by zero
$endgroup$
– Xiaomi
Oct 1 '18 at 1:45
$begingroup$
@Xiaomi I improved my answer. Is that correct now? And how to answer in a single statement?
$endgroup$
– Yibei He
Oct 1 '18 at 1:52
$begingroup$
@YibeiHe, still no. $f_X(x) ne P(X=x)$
$endgroup$
– Zamarion
Oct 1 '18 at 1:54
add a comment |
$begingroup$
No. You should answer in a single statement, not a case by case basis. Secondly, your proof is wrong because $P(X = x)$ is $0$ since $X$ is a continuous random variable, so you're dividing by zero
$endgroup$
– Xiaomi
Oct 1 '18 at 1:45
$begingroup$
@Xiaomi I improved my answer. Is that correct now? And how to answer in a single statement?
$endgroup$
– Yibei He
Oct 1 '18 at 1:52
$begingroup$
@YibeiHe, still no. $f_X(x) ne P(X=x)$
$endgroup$
– Zamarion
Oct 1 '18 at 1:54
$begingroup$
No. You should answer in a single statement, not a case by case basis. Secondly, your proof is wrong because $P(X = x)$ is $0$ since $X$ is a continuous random variable, so you're dividing by zero
$endgroup$
– Xiaomi
Oct 1 '18 at 1:45
$begingroup$
No. You should answer in a single statement, not a case by case basis. Secondly, your proof is wrong because $P(X = x)$ is $0$ since $X$ is a continuous random variable, so you're dividing by zero
$endgroup$
– Xiaomi
Oct 1 '18 at 1:45
$begingroup$
@Xiaomi I improved my answer. Is that correct now? And how to answer in a single statement?
$endgroup$
– Yibei He
Oct 1 '18 at 1:52
$begingroup$
@Xiaomi I improved my answer. Is that correct now? And how to answer in a single statement?
$endgroup$
– Yibei He
Oct 1 '18 at 1:52
$begingroup$
@YibeiHe, still no. $f_X(x) ne P(X=x)$
$endgroup$
– Zamarion
Oct 1 '18 at 1:54
$begingroup$
@YibeiHe, still no. $f_X(x) ne P(X=x)$
$endgroup$
– Zamarion
Oct 1 '18 at 1:54
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
$$P(X le Y mid min{X, Y} = x) = frac{P(X le Y, min{X,Y} in [x, x+dx])}{P(min{X,Y} in [x, x+dx])}.$$
The numerator is equal to
$$P(X le Y, X in [x, x + dx]) = P(X in [x, x+dx]) P(Y ge x) = (lambda e^{-lambda x} , dx)(e^{-mu x}).$$
The denominator is equal to
$$P(X le Y, X in [x, x + dx]) + P(X ge Y, Y in [x, x + dx])
= (lambda e^{-lambda x} , dx)(e^{-mu x}) + (mu e^{-mu x} , dx) (e^{-lambda x})$$
by similar computations.
Thus the answer is $frac{lambda}{lambda + mu}$. Note that it is interesting that the answer does not depend on $x$. [You could also quickly answer this question using properties of Poisson processes, but perhaps this is off-topic.]
Response to comment: It actually does work. $$F(x + dx) - F(x) = e^{-lambda x} - e^{-lambda (x + dx)} = e^{-lambda x}(1 - e^{-lambda , dx}).$$
Then using $e^z = 1 + z + O(z^2)$, we can plug in $e^{-lambda dx} = 1 - lambda , dx$ to obtain $lambda e^{-lambda x} , dx$.
$endgroup$
$begingroup$
Thanks a lot! One question: if I do the part P(x=[x,x+dx]) into F(x+dx)-F(x), it doesn’t work. Why?
$endgroup$
– Yibei He
Oct 1 '18 at 2:28
$begingroup$
@YibeiHe See my edit.
$endgroup$
– angryavian
Oct 1 '18 at 5:56
add a comment |
$begingroup$
The second part isn't quite right, since $P(X=x)=0$ for any continuous random variable $X$.
You can compute this probability that way,
begin{align}
P(X<Y|X=x) &= P(x<Y) \
&= 1 - int_0^xmu e^{-mu y}dy \
&= 1 - (1-e^{-mu x}) \
&= e^{mu x}
end{align}
Note that this is not the final answer and you should be careful on how you have split your two cases.
$endgroup$
$begingroup$
Shall I divide it by the pdf of X since the condition is X=x.
$endgroup$
– Yibei He
Oct 1 '18 at 1:54
add a comment |
Your Answer
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2 Answers
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2 Answers
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$begingroup$
$$P(X le Y mid min{X, Y} = x) = frac{P(X le Y, min{X,Y} in [x, x+dx])}{P(min{X,Y} in [x, x+dx])}.$$
The numerator is equal to
$$P(X le Y, X in [x, x + dx]) = P(X in [x, x+dx]) P(Y ge x) = (lambda e^{-lambda x} , dx)(e^{-mu x}).$$
The denominator is equal to
$$P(X le Y, X in [x, x + dx]) + P(X ge Y, Y in [x, x + dx])
= (lambda e^{-lambda x} , dx)(e^{-mu x}) + (mu e^{-mu x} , dx) (e^{-lambda x})$$
by similar computations.
Thus the answer is $frac{lambda}{lambda + mu}$. Note that it is interesting that the answer does not depend on $x$. [You could also quickly answer this question using properties of Poisson processes, but perhaps this is off-topic.]
Response to comment: It actually does work. $$F(x + dx) - F(x) = e^{-lambda x} - e^{-lambda (x + dx)} = e^{-lambda x}(1 - e^{-lambda , dx}).$$
Then using $e^z = 1 + z + O(z^2)$, we can plug in $e^{-lambda dx} = 1 - lambda , dx$ to obtain $lambda e^{-lambda x} , dx$.
$endgroup$
$begingroup$
Thanks a lot! One question: if I do the part P(x=[x,x+dx]) into F(x+dx)-F(x), it doesn’t work. Why?
$endgroup$
– Yibei He
Oct 1 '18 at 2:28
$begingroup$
@YibeiHe See my edit.
$endgroup$
– angryavian
Oct 1 '18 at 5:56
add a comment |
$begingroup$
$$P(X le Y mid min{X, Y} = x) = frac{P(X le Y, min{X,Y} in [x, x+dx])}{P(min{X,Y} in [x, x+dx])}.$$
The numerator is equal to
$$P(X le Y, X in [x, x + dx]) = P(X in [x, x+dx]) P(Y ge x) = (lambda e^{-lambda x} , dx)(e^{-mu x}).$$
The denominator is equal to
$$P(X le Y, X in [x, x + dx]) + P(X ge Y, Y in [x, x + dx])
= (lambda e^{-lambda x} , dx)(e^{-mu x}) + (mu e^{-mu x} , dx) (e^{-lambda x})$$
by similar computations.
Thus the answer is $frac{lambda}{lambda + mu}$. Note that it is interesting that the answer does not depend on $x$. [You could also quickly answer this question using properties of Poisson processes, but perhaps this is off-topic.]
Response to comment: It actually does work. $$F(x + dx) - F(x) = e^{-lambda x} - e^{-lambda (x + dx)} = e^{-lambda x}(1 - e^{-lambda , dx}).$$
Then using $e^z = 1 + z + O(z^2)$, we can plug in $e^{-lambda dx} = 1 - lambda , dx$ to obtain $lambda e^{-lambda x} , dx$.
$endgroup$
$begingroup$
Thanks a lot! One question: if I do the part P(x=[x,x+dx]) into F(x+dx)-F(x), it doesn’t work. Why?
$endgroup$
– Yibei He
Oct 1 '18 at 2:28
$begingroup$
@YibeiHe See my edit.
$endgroup$
– angryavian
Oct 1 '18 at 5:56
add a comment |
$begingroup$
$$P(X le Y mid min{X, Y} = x) = frac{P(X le Y, min{X,Y} in [x, x+dx])}{P(min{X,Y} in [x, x+dx])}.$$
The numerator is equal to
$$P(X le Y, X in [x, x + dx]) = P(X in [x, x+dx]) P(Y ge x) = (lambda e^{-lambda x} , dx)(e^{-mu x}).$$
The denominator is equal to
$$P(X le Y, X in [x, x + dx]) + P(X ge Y, Y in [x, x + dx])
= (lambda e^{-lambda x} , dx)(e^{-mu x}) + (mu e^{-mu x} , dx) (e^{-lambda x})$$
by similar computations.
Thus the answer is $frac{lambda}{lambda + mu}$. Note that it is interesting that the answer does not depend on $x$. [You could also quickly answer this question using properties of Poisson processes, but perhaps this is off-topic.]
Response to comment: It actually does work. $$F(x + dx) - F(x) = e^{-lambda x} - e^{-lambda (x + dx)} = e^{-lambda x}(1 - e^{-lambda , dx}).$$
Then using $e^z = 1 + z + O(z^2)$, we can plug in $e^{-lambda dx} = 1 - lambda , dx$ to obtain $lambda e^{-lambda x} , dx$.
$endgroup$
$$P(X le Y mid min{X, Y} = x) = frac{P(X le Y, min{X,Y} in [x, x+dx])}{P(min{X,Y} in [x, x+dx])}.$$
The numerator is equal to
$$P(X le Y, X in [x, x + dx]) = P(X in [x, x+dx]) P(Y ge x) = (lambda e^{-lambda x} , dx)(e^{-mu x}).$$
The denominator is equal to
$$P(X le Y, X in [x, x + dx]) + P(X ge Y, Y in [x, x + dx])
= (lambda e^{-lambda x} , dx)(e^{-mu x}) + (mu e^{-mu x} , dx) (e^{-lambda x})$$
by similar computations.
Thus the answer is $frac{lambda}{lambda + mu}$. Note that it is interesting that the answer does not depend on $x$. [You could also quickly answer this question using properties of Poisson processes, but perhaps this is off-topic.]
Response to comment: It actually does work. $$F(x + dx) - F(x) = e^{-lambda x} - e^{-lambda (x + dx)} = e^{-lambda x}(1 - e^{-lambda , dx}).$$
Then using $e^z = 1 + z + O(z^2)$, we can plug in $e^{-lambda dx} = 1 - lambda , dx$ to obtain $lambda e^{-lambda x} , dx$.
edited Oct 1 '18 at 5:56
answered Oct 1 '18 at 1:55
angryavianangryavian
42.1k23381
42.1k23381
$begingroup$
Thanks a lot! One question: if I do the part P(x=[x,x+dx]) into F(x+dx)-F(x), it doesn’t work. Why?
$endgroup$
– Yibei He
Oct 1 '18 at 2:28
$begingroup$
@YibeiHe See my edit.
$endgroup$
– angryavian
Oct 1 '18 at 5:56
add a comment |
$begingroup$
Thanks a lot! One question: if I do the part P(x=[x,x+dx]) into F(x+dx)-F(x), it doesn’t work. Why?
$endgroup$
– Yibei He
Oct 1 '18 at 2:28
$begingroup$
@YibeiHe See my edit.
$endgroup$
– angryavian
Oct 1 '18 at 5:56
$begingroup$
Thanks a lot! One question: if I do the part P(x=[x,x+dx]) into F(x+dx)-F(x), it doesn’t work. Why?
$endgroup$
– Yibei He
Oct 1 '18 at 2:28
$begingroup$
Thanks a lot! One question: if I do the part P(x=[x,x+dx]) into F(x+dx)-F(x), it doesn’t work. Why?
$endgroup$
– Yibei He
Oct 1 '18 at 2:28
$begingroup$
@YibeiHe See my edit.
$endgroup$
– angryavian
Oct 1 '18 at 5:56
$begingroup$
@YibeiHe See my edit.
$endgroup$
– angryavian
Oct 1 '18 at 5:56
add a comment |
$begingroup$
The second part isn't quite right, since $P(X=x)=0$ for any continuous random variable $X$.
You can compute this probability that way,
begin{align}
P(X<Y|X=x) &= P(x<Y) \
&= 1 - int_0^xmu e^{-mu y}dy \
&= 1 - (1-e^{-mu x}) \
&= e^{mu x}
end{align}
Note that this is not the final answer and you should be careful on how you have split your two cases.
$endgroup$
$begingroup$
Shall I divide it by the pdf of X since the condition is X=x.
$endgroup$
– Yibei He
Oct 1 '18 at 1:54
add a comment |
$begingroup$
The second part isn't quite right, since $P(X=x)=0$ for any continuous random variable $X$.
You can compute this probability that way,
begin{align}
P(X<Y|X=x) &= P(x<Y) \
&= 1 - int_0^xmu e^{-mu y}dy \
&= 1 - (1-e^{-mu x}) \
&= e^{mu x}
end{align}
Note that this is not the final answer and you should be careful on how you have split your two cases.
$endgroup$
$begingroup$
Shall I divide it by the pdf of X since the condition is X=x.
$endgroup$
– Yibei He
Oct 1 '18 at 1:54
add a comment |
$begingroup$
The second part isn't quite right, since $P(X=x)=0$ for any continuous random variable $X$.
You can compute this probability that way,
begin{align}
P(X<Y|X=x) &= P(x<Y) \
&= 1 - int_0^xmu e^{-mu y}dy \
&= 1 - (1-e^{-mu x}) \
&= e^{mu x}
end{align}
Note that this is not the final answer and you should be careful on how you have split your two cases.
$endgroup$
The second part isn't quite right, since $P(X=x)=0$ for any continuous random variable $X$.
You can compute this probability that way,
begin{align}
P(X<Y|X=x) &= P(x<Y) \
&= 1 - int_0^xmu e^{-mu y}dy \
&= 1 - (1-e^{-mu x}) \
&= e^{mu x}
end{align}
Note that this is not the final answer and you should be careful on how you have split your two cases.
edited Oct 1 '18 at 2:28
answered Oct 1 '18 at 1:47
ZamarionZamarion
33719
33719
$begingroup$
Shall I divide it by the pdf of X since the condition is X=x.
$endgroup$
– Yibei He
Oct 1 '18 at 1:54
add a comment |
$begingroup$
Shall I divide it by the pdf of X since the condition is X=x.
$endgroup$
– Yibei He
Oct 1 '18 at 1:54
$begingroup$
Shall I divide it by the pdf of X since the condition is X=x.
$endgroup$
– Yibei He
Oct 1 '18 at 1:54
$begingroup$
Shall I divide it by the pdf of X since the condition is X=x.
$endgroup$
– Yibei He
Oct 1 '18 at 1:54
add a comment |
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$begingroup$
No. You should answer in a single statement, not a case by case basis. Secondly, your proof is wrong because $P(X = x)$ is $0$ since $X$ is a continuous random variable, so you're dividing by zero
$endgroup$
– Xiaomi
Oct 1 '18 at 1:45
$begingroup$
@Xiaomi I improved my answer. Is that correct now? And how to answer in a single statement?
$endgroup$
– Yibei He
Oct 1 '18 at 1:52
$begingroup$
@YibeiHe, still no. $f_X(x) ne P(X=x)$
$endgroup$
– Zamarion
Oct 1 '18 at 1:54