# S2 help, think book may be wrong.Watch

#1
Good evening all. I'm revising for my stats exam and I've come across a question which I think may have been wrote wrong. It's Normal Distribution stuff.

Question:
A machine set to produce metal discs of diameter 11.90cm has an annual service. After the service, a random sample of 36 discs is measured and found to have a mean diameter of 11.930 cm and gives an estimated population variance of 0.072 cm^2. Test, at the 1% significance level, whether the machine is now producing discs of mean diameter greater than 11.90 cm.

For reference, 1% is 2.326 sds away from the mean.

Thanks
0
11 years ago
#2
Errrr, what do you get?

We're not going to blindly do it for you.

Been a while since I've done this, You set H0 and H1 where H0 is there is no change and H1 is the mean is greater than 11.930

So then you do (O-E)/v And if this is in the critical area you reject H0 and accept H1 but if not you accept H0 and reject H1?
0
#3

Well when I use the variance as 0.072 i get:

(11.930-11.9)/√(0.072/36) = 0.6708

And 1% range being 2.326, shows that it isn't. However book says 2.5, which leads me to believe that 0.072 is not the variance, but the standard deviation.
As from our equation above:

(11.930-11.9)/(0.072/√36) = 2.5

What do you reckon?
0
11 years ago
#4
Yeh, that seems right. And 0.6708 is far too low really compared to the critical region. I would be suspcious anyways.
0
#5
Great. Thanks Totally Tom. Does anyone else agree?
0
3 years ago
#6
I will agree with you even though this thread was started 8 years ago. Thanks for helping point out the flaws in the textbook
0
X

new posts
Latest
My Feed

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

see more

### See more of what you like onThe Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

### University open days

• Bournemouth University
Wed, 31 Jul '19
• Staffordshire University
Wed, 7 Aug '19
• University of Derby
Foundation Open Event Further education
Wed, 7 Aug '19

### Poll

Join the discussion

#### What are your favourite crisps?

Kettle Chips (43)
16.67%
McCoys Salt and Vinegar (19)
7.36%
McCoys Flame Grilled Steak (17)
6.59%
Walkers Prawn Cockail (28)
10.85%
Monster Munch (19)
7.36%
Pringles (70)
27.13%
Walkers Oven Baked Crisps (15)
5.81%
Walkers Beef and Onion (3)
1.16%
Thai Sweet Chili Sensations (44)
17.05%