Take the 2-minute tour ×
Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

Let $P(X=3)=0.4$ and $P(Y=2)=0.5$. I need to find $P(X=3,Y=2)$.

I'm thinking that I ought to just multiply the two probabilities: $P(X=3) \times P(Y=2)$ to get $0.4 \times 0.5 = 0.2$, but is this correct?

If not, how do I go about finding this? There is a chance that it is uncomputable.

Edit: They are not independent.

Both X and Y are random variables.

share|improve this question
    
If you assume that X and Y are independent random variables, then your answer is correct. Otherwise, we would need a bit more information to answer your question. –  Cristian Oct 9 '12 at 17:17
2  
The answer can be anything between 0.4 and zero if independence is not assumed. –  Harald Hanche-Olsen Oct 9 '12 at 17:18
    
I checked, and they are not independent. –  jonelliot Oct 9 '12 at 17:20
2  
@JeremyQuick: How do they depend on each other? –  Thomas Oct 9 '12 at 17:26
1  
For instance, do you know the likelihood of Y given X. If so, you could compute $P(X \cap Y)=P(Y|X)P(X)$. But without any further information, it is difficult to recover the joint probability from the marginals. –  Cristian Oct 9 '12 at 17:26
show 1 more comment

1 Answer 1

up vote 1 down vote accepted

If you assume that X and Y are independent random variables, then your answer is correct. Otherwise, we would need a bit more information to answer your question.

share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.