In: Statistics and Probability
The random variable X can take on the values 1, 2 and 3 and the random variable Y can take on the values 1, 3, and 4. The joint probability distribution of X and Y is given in the following table:
Y |
||||
1 |
3 |
4 |
||
X |
1 |
0.1 |
0.15 |
0.1 |
2 |
0.1 |
0.1 |
0.1 |
|
3 |
0.1 |
0.2 |
a. What value should go in the blank cell?
b. Describe in words and notation the event that has probability 0.2 in the table.
c. Calculate the marginal distribution of X and the marginal distribution of Y.
d. Are X and Y independent events? Show why or why not with calculations.
e. Calculate the conditional distribution of X given Y=1.
f. Calculate E(X) and E(Y).
g. Calculate V(X) and V(Y).
h. Calculate E(X|Y=1).
Solution-:
X and Y be the two random variable with possible values 1,2,3 and 1,3,4 respectively.
The joint probability distribution of X and Y is given in the following table:
Y | ||||
X\Y | 1 | 3 | 4 | |
1 | 0.1 | 0.15 | 0.1 | |
X | 2 | 0.1 | 0.1 | 0.1 |
3 | a | 0.1 | 0.2 |
(a) we find as follows;
We know that two conditions (i) and (ii)
We preare following table :
X\Y | 1 | 3 | 4 | Total |
1 | 0.1 | 0.15 | 0.1 | 0.35 |
2 | 0.1 | 0.1 | 0.1 | 0.3 |
3 | 0.05 | 0.1 | 0.2 | 0.35 |
Total | 0.25 | 0.35 | 0.4 | 1 |
We get, a=0.05
(b) The event that has probability 0.2 in the table means probability of X=3 and Y=4.
i.e.
(c) The marginal probability of X is,
X | 1 | 2 | 3 | Total |
P(x) | 0.35 | 0.3 | 0.35 | 1 |
The marginal probabilty of Y is,
Y | 1 | 3 | 4 | Total |
P(y) | 0.25 | 0.35 | 0.4 | 1 |
(d) X and Y are called independent random variables iff ,
In other words
for all i and j.
Here,
Simalarly we find all probabilitie, condition of independecy not holds;
Here, we senen that X and Y are not independent.
(e) The conditional distribution of X given Y=1.
Total | ||||
Y=1 | 0.1 | 0.1 | 0.05 | 0.25 |
The conditional probability of X given Y=1 ,
X | 1 | 2 | 3 | Total |
P(X/Y=1) | 0.4 | 0.4 | 0.2 | 1 |
(f) We find , E(X),E(Y)
and
(g) We find , V(X),V(Y)
Where,
and
Where,