# Binomial expectation E(X|X>1)

#### AliceBaker

##### New Member
General case: X~B(n,p)
I could integrate the product of x and pmf from 1 to infinity and divide it by the integral of just pmf from 1 to infinity to find it, but I'm really stuck since I can't do it due to the sophisticated pmf function of binomial.
How can we find this expectation and Variance(X|X>1)?
Thank u

#### katxt

##### Member
Seeing it's binomial, it is a sum to n terms, rather than an integral to infinity. Imagine the full probability table. Each x has a p, xp and x^2p. From these you can work out mean = sum of xp, and var = sum of x^2p - mean^2.
Now we have x = 0 and x = 1 removed. For x=0 the xp and x^2p were 0 and 0, Find the values for xp and x^2p for x = 1. Find the new sum of xp and sum of x^2p and so your new mean and variance.