View Full Version : Binomial or Geometric distribution?

02-03-2011, 10:41 PM
Can someone please help me out with this. I've been trying for about forty minutes, but I'm not getting this. Trust me, I've done everything I can to try to look up strategies/tips on these types of problems, but it's not helping. I REALLY appreciate any assistance!

32% of employed women have never been married. Suppose you randomly select 10 employed women and determine marital history.

1) What is the random var X of interest here? Define X. Is X binomial, geometric, or normal?
2) If 10 employed women are selected at random, what is the prb that 2 or fewer have never been married?
3) What are the mean and standard deviation of X?
4) Find the prb that the # of employed women who have never been married is within 1 standard deviation of its mean.

70% of Americans are overweight. Suppose that a # randomly selected are weighed.

5) Find the prb that 18 or more of the 30 students in a class are overweight
6) How many Americans would you expect to weigh before you encounter the first overweight individual?
7) What is the prb that it takes more than 5 attempts before an overweight person is found?

Thank you so so so so much if you could help! I really appreciate it!

02-03-2011, 11:02 PM
Are you asking sequentially and waiting with excitement to see when you find the first married? This might be Geometric. Check out #6 or #7 and see what that sounds like.

You're looking at a chosen population,

p = 0.32 -- Never married
q = 1 - 0.32 = 0.68 = -- Married
n = 10

It doesn't get any more Binomial than that. (except for 6 & 7)

02-03-2011, 11:11 PM
The first group of questions if binomial, and the second set is geometic. Thanks, I've gotten that far so far.

I really don't know how to apply those numbers to the questions, though. I'm sorry, I usually keep up very well with stat. This topic is just a bit confusing for me.

02-04-2011, 12:37 AM
No, #5 is Binomial

If there were four, rather than 10.

(p+q)^4 = p^4 + 4(p^3)q + 6(p^2)(q^2) + 4p(q^3) + q^4

What is the probability that 2 or more have been married? 6(p^2)(q^2) + 4p(q^3) + q^4
What is the probability only one has married? 4(p^3)q

May such questions can be answered.