# Poisson Distribution Question

• Jul 21st 2010, 03:15 PM
timmcan
Poisson Distribution Question
So I got a summer homework package from school (Doh), and I need to teach myself Poisson Distribution before school starts. So here's the question I've been stuck on for a while. (I need help on b.)

The number of faults in glass sheets occur at a rate of 2.1 per sq. metre. If a 1x1 sq. m glass sheet contains at least 3 faults it is returned to the manufacturer.

a) Find the probability that a 1x1 sq. m sheet is returned to the manufacturer.

$P(X > 2) = 1 - P(0) - P(1) - P(2)$
$P(X > 2) = 1 - e^{-2.1}(1+2.1+\frac{2.1^{2}}{2})$
$P(X > 2) = 0.3504$

b) Six such glass sheets are inspected. What is the probability that at least half of them are returned to the manufacturer? (Here's the answer provided: 0.6817)

It would be nice if you can show me the work and proper notation, because I barely have a clue.

Thanks.
• Jul 21st 2010, 04:20 PM
Danneedshelp
Let $Y$ be a random varibale that represents the number defective glass sheets. Then, $Y$ is $Binomial(n=6, p=0.3504)$, since the probability of a success is equal to the probability of a sheet of glass being defective. Thus, the probability that at least half the glass sheets will be defective out of a batch of 6 glass sheets ought to be

$P(Y\geq\\2)=\sum_{y=3}^{6}{{6}\choose{y}}(0.3504)^ {y}(1-0.3504)^{6-y}$.

I think that's correct.
• Jul 23rd 2010, 06:45 AM
timmcan

I see what you are doing using binomial distribution. However, the answer that your equation got wasn't the answer that was provided. The answer that you got was about 0.353.

I also used the Poisson Distribution way to solve the question based on your logic, and it got the same (or approx.) answer:

$\lambda = (0.3504)(6) = 2.124$

so,

$P(X > 2) = 1 - P(0) - P(1) - P(2)$
$P(X > 2) = 1 - e^{-2.124}(1 + 2.124 + \frac{2.124^{2}}{2})$
$P(X > 2) = 0.3568$

Hm.. are we doing something wrong or the answer provided is just wrong?
• Jul 23rd 2010, 07:14 AM
SpringFan25
Danneedshelp is correct,

$Y \sim Binomial(n=6, p=0.3504)$

$P(Y \geq 2) = 0.681689$

You can use Binomial Distribution: Probability Calculator to check your answers when calculating binomial probabilities.
• Jul 23rd 2010, 07:20 AM
Quote:

Originally Posted by timmcan
So I got a summer homework package from school (Doh), and I need to teach myself Poisson Distribution before school starts. So here's the question I've been stuck on for a while. (I need help on b.)

The number of faults in glass sheets occur at a rate of 2.1 per sq. metre. If a 1x1 sq. m glass sheet contains at least 3 faults it is returned to the manufacturer.

a) Find the probability that a 1x1 sq. m sheet is returned to the manufacturer.

$P(X > 2) = 1 - P(0) - P(1) - P(2)$
$P(X > 2) = 1 - e^{-2.1}(1+2.1+\frac{2.1^{2}}{2})$
$P(X > 2) = 0.3504$

b) Six such glass sheets are inspected. What is the probability that at least half of them are returned to the manufacturer? (Here's the answer provided: 0.6817)

It would be nice if you can show me the work and proper notation, because I barely have a clue.

Thanks.

For (b), it looks that the answer provided is the probability when only 4 glass sheets are inspected.
• Jul 23rd 2010, 07:36 AM
timmcan
Thanks for the link SpringFan, but Danneedshelp used $P(X \geq 3)$, not $P(X \geq 2)$. Still, it seems that what you did came up as the correct answer according to the answer sheet. I guess there's just something wrong with the wording of the question.