# Finding pmf with an incomplete table

• Nov 2nd 2009, 12:48 PM
brenden720
Finding pmf with an incomplete table
Hey,
I just had a quick question about this problem I had on my midterm recently. It was: Consider the following (incomplete) description of a discrete random variable X:
Outcome p(x) = P(X = x) F(x) = P(X</=x)
X = 1----------(blank)------------(blank)
X = 2-----------(0.2)---------------(0.2)
X = 4----------(blank)--------------(0.4)
X = 6-----------(0.5)--------------(blank)
X = 8-----------(0.1)--------------(blank)

It seems the table gets messed up when saving. The 0.2, 0.5, and 0.1 should be under the p(x) column and the 0.2 and 0.4 should be under the F(x) column
a) Fill in the missing numbers in the above table.

I'm not sure how to fill out the column for p(x) = P(X = x).
If I use the formula for the binomial distribution I don't get the correct answers. For X = 1 it should be 0, and for X = 4 it should be 0.2.
Do I use the binomial distribution formula for this? Or is there a different way? Since I don't know the number of attempts there were, I'm slightly confused. The information stated above was all that was given.
• Nov 2nd 2009, 03:23 PM
mr fantastic
Quote:

Originally Posted by brenden720
Hey,
I just had a quick question about this problem I had on my midterm recently. It was: Consider the following (incomplete) description of a discrete random variable X:
Outcome p(x) = P(X = x) F(x) = P(X</=x)
X = 1----------(blank)------------(blank)
X = 2-----------(0.2)---------------(0.2)
X = 4----------(blank)--------------(0.4)
X = 6-----------(0.5)--------------(blank)
X = 8-----------(0.1)--------------(blank)

It seems the table gets messed up when saving. The 0.2, 0.5, and 0.1 should be under the p(x) column and the 0.2 and 0.4 should be under the F(x) column
a) Fill in the missing numbers in the above table.

I'm not sure how to fill out the column for p(x) = P(X = x).
If I use the formula for the binomial distribution I don't get the correct answers. For X = 1 it should be 0, and for X = 4 it should be 0.2.
Do I use the binomial distribution formula for this? Or is there a different way? Since I don't know the number of attempts there were, I'm slightly confused. The information stated above was all that was given.
1. $\Pr(X \leq 2) = 0.2$.
2. $\Pr(X \leq 2) = \Pr(X = 1) + \Pr(X = 2) = \Pr(X = 1) + 0.2$.
3. From 1. and 2. it follows that $\Pr(X = 1) = ....$.