1. ## Finding Standard Deviation

Hello everyone. I am new here, so excuse me if I sound a little too noobish. I am currently faced with the current problem on my homework. It isn't urgent, but finals are coming up and this is one thing I'm really struggling with.

Johnson Electronics makes calculators. Consumer satisfaction is one of the top priorities of the company's management. The company guarantees the refund of money or a replacement for any calculator that malfunctions within two years from the data of purchase. It is known from past data that despite all efforts, 5% of the calculators manufactured by this company malfunction within a 2-year period. The company recently mailed 500 such calculators to its customers.

Find the probability that exactly 29 of the 500 calculators will be returned for refund or replacement within a 2-year period.

So from the given, I have concluded:
h0:u=.05, h1:u>.05 (Not positive these are right)
n=500, xbar=.058

I cannot determine the value of the standard deviation (will be referred to as s).

The formula I believe I need to use for this equation is:
Z=(xbar-u0)/(s/sqrt(n))
Then using the chart, figure out what the P value is.

I will attempt to elaborate further upon request.

Kroger

2. ## Re: Finding Standard Deviation

h0:u=.05, h1:u>.05 (Not positive these are right)
Null (H0's) hypotheses are not written in terms of alpha.

The formula I believe I need to use for this equation is:
Z=(xbar-u0)/(s/sqrt(n))
Then using the chart, figure out what the P value is.
This sounds pretty good to me. Go ahead and do it and let us know what you get.

3. ## Re: Finding Standard Deviation

Originally Posted by trinker
Null (H0's) hypotheses are not written in terms of alpha.

This sounds pretty good to me. Go ahead and do it and let us know what you get.
I was referring the .05 being the 5% in the stated problem.
For the equation I have, I cannot figure out how to determine what the standard deviation is. This is where I am stuck.

4. ## Re: Finding Standard Deviation

Originally Posted by trinker
Null (H0's) hypotheses are not written in terms of alpha.
I'm thinking that their 5% is coming from the failure percentage of the population actually.

My main concern with that is that it seems like this is a test of proportion and not a test of the mean (although really it's the same thing sometimes people can get hung up on those small details).

5. ## Re: Finding Standard Deviation

Originally Posted by Dason
I'm thinking that their 5% is coming from the failure percentage of the population actually.
That is my intention. I am unsure if that is correct or not.

6. ## Re: Finding Standard Deviation

Not sure how this question is related to hypothesis testing. I think it is just a simple question asking the probability related to a Binomial distribution.

7. ## Re: Finding Standard Deviation

Originally Posted by BGM
Not sure how this question is related to hypothesis testing. I think it is just a simple question asking the probability related to a Binomial distribution.
So I am actually way off cue here? Not to try to get pity or anything, but I missed like 5 classes (mainly covering this chapter) due to the death of my grandmother. So I am way behind and very confused.

8. ## Re: Finding Standard Deviation

This is because you're comparing two proportions. Different formula.

EDIT: I see Dason has already given you that clue.

9. ## Re: Finding Standard Deviation

So I am actually way off cue here? Not to try to get pity or anything, but I missed like 5 classes (mainly covering this chapter) due to the death of my grandmother. So I am way behind and very confused.
http://stattrek.com/ap-statistics-4/...roportion.aspx

Sorry to hear that. Hope this catches you up.

10. ## Re: Finding Standard Deviation

Originally Posted by trinker
EDIT: I see Dason has already given you that clue.[/COLOR]
If I did it was an accident. I didn't really read the original post thoroughly. It was more of a comment that typically we would frame the hypothesis in terms of the population. But really reading over the problem I don't really think there should be a hypothesis test at all - which is what BGM is hinting at.

11. ## Re: Finding Standard Deviation

Okay, so s=sqrt((.05-.058)^2/500)=.0003578?

12. ## Re: Finding Standard Deviation

Why not a proportion test dason?

EDIT: PhatKroger10 That web link was for comparing two proportions this is the page that explains your problem( Sorry for the mix up):
http://stattrek.com/Lesson5/Proportion.aspx?Tutorial=AP

13. ## Re: Finding Standard Deviation

Mainly because it seems that the actual question is just:
Find the probability that exactly 29 of the 500 calculators will be returned for refund or replacement within a 2-year period.

14. ## Re: Finding Standard Deviation

Originally Posted by dason
If I did it was an accident. I didn't really read the original post thoroughly. It was more of a comment that typically we would frame the hypothesis in terms of the population. But really reading over the problem I don't really think there should be a hypothesis test at all - which is what BGM is hinting at.
yeah: (TS requires I have a certain number of characters to post)

Originally Posted by Dason
My main concern with that is that it seems like this is a test of proportion and not a test of the mean (although really it's the same thing sometimes people can get hung up on those small details)

15. ## Re: Finding Standard Deviation

So with my current Z formula:
(.058-.05)/(.00036/sqrt(500))= 496.90

That does not seem right haha.

EDIT: Just saw your edit, I will check that link out and then attempt to solve my question and reword if necessary.