# binomial probability

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• Sep 22nd 2008, 11:07 PM
brumby_3
binomial probability
- Find the binomial probability:

P(25 < B(45, 0.2) < 35)

I tried:
=BINOMDIST(34,45,0.2,1)-BINOMDIST(26,45,0.2,1)
And it comes out as error. Can someone please tell me what I'm doing wrong.

__________________

In a new sheet, enter the numbers 0 through
n = 30 in column A. Type 0 in A1, 1
in
A2 and 2 in A3, then highlight the three cells, and drag the small square in the
bottom right corner until cell
A31 which should read 30. In cell B1 calculate P(X = 0)
where
X is a binomial random variable with n = 30 and p = 0.4 . Use the command

=BINOMDIST(A1,30,0.4,FALSE). Copy down to cell B31.

Can someone please explain to me what I do in cell B1? Like what exactly am I supposed to type?
• Sep 22nd 2008, 11:41 PM
CaptainBlack
Quote:

Originally Posted by brumby_3
- Find the binomial probability:

P(25 < B(45, 0.2) < 35)

I tried:
=BINOMDIST(34,45,0.2,1)-BINOMDIST(26,45,0.2,1)
And it comes out as error. Can someone please tell me what I'm doing wrong.

__________________

In a new sheet, enter the numbers 0 through
n = 30 in column A. Type 0 in A1, 1
in A2 and 2 in A3, then highlight the three cells, and drag the small square in the
bottom right corner until cell A31 which should read 30. In cell B1 calculate P(X = 0)
where X is a binomial random variable with n = 30 and p = 0.4 . Use the command
=BINOMDIST(A1,30,0.4,FALSE). Copy down to cell B31.

Can someone please explain to me what I do in cell B1? Like what exactly am I supposed to type?

The fourth parameter in BINOMDIS should be TRUE for the cumulative distribution.

RonL

• Sep 22nd 2008, 11:49 PM
brumby_3
it still doesn't work. I get the same answer which is 4.92803E-09

?????? lol

And same with the second half of my question - I keep getting a funny looking number with an E in it.
• Sep 22nd 2008, 11:58 PM
CaptainBlack
Quote:

Originally Posted by brumby_3
it still doesn't work. I get the same answer which is 4.92803E-09

?????? lol

And same with the second half of my question - I keep getting a funny looking number with an E in it.

Ahh.. you said it comes out as error, not that you thought the answer was wrong!

What makes you think that that is wrong?

The mean number of successes in 45 trials with p=0.2 is 9, 25 is a long way above 9, so far in fact that for most purposes the cumulative probability of 25 or fewer successes is indistinguishable from 1.

(Note BINOMDIS runs out of prescission before you get to 34 triaks in this case, and the cumulative returned is greater than 1, but that should not metter)

Also:

P(25 < B(45, 0.2) < 35)=Q(34)-Q(25)

where Q denote the cumulative distribution, not:

P(25 < B(45, 0.2) < 35)=Q(34)-Q(26)

which is what you have.

RonL
• Sep 23rd 2008, 12:02 AM
brumby_3
So it's right? Even with the E- in it? O_O
• Sep 23rd 2008, 12:07 AM
CaptainBlack
Quote:

Originally Posted by brumby_3
it still doesn't work. I get the same answer which is 4.92803E-09

?????? lol

And same with the second half of my question - I keep getting a funny looking number with an E in it.

That is not an error it is standard exponential notation (same as most calculators and all numerical programming languages):

4.92803E-09 means $\displaystyle 4.92803 \times 10^{-9}$

RonL