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# Official Edexcel S3 thread Wednesday 20 May 2015 Morning [6691] watch

1. Can anyone explain why the question 'Explain what you understand by the central limit theorem' has come up twice with one being 2 marks and the other being 3 marks? Also what is the perfect definition to always pick up full marks
2. (Original post by STATER)
But it is still an assumption? To be able to carry out the hypothesis test you must assume a normal distribution.
well, yes, technically, it allows us to assume the means are normally distributed, and that's why we use it/why it's important. but the questions asks "State two assumptions you made in carrying out the test" so to carry out the test first we need to assume the conditions to enable us to use CLT

i hope that makes sense :P
3. In Soloman paper D q5A Why are they multiplying the probability by 2 in the markscheme?
4. Which solomon papers has everyone done?
5. (Original post by kidder007)
Can anyone explain why the question 'Explain what you understand by the central limit theorem' has come up twice with one being 2 marks and the other being 3 marks? Also what is the perfect definition to always pick up full marks
The only difference between two or three marks is the mention of ANY distribution the three marks one is the perfect definition

Perfect definition: For a large sample(X1+...Xn) for any distribution can be modelled as X~N(mean,variance/number of sample)
6. (Original post by mmms95)
well, yes, technically, it allows us to assume the means are normally distributed, and that's why we use it/why it's important. but the questions asks "State two assumptions you made in carrying out the test" so to carry out the test first we need to assume the conditions to enable us to use CLT

i hope that makes sense :P
I see what you mean, but the question is too vague in that case. I don't see why my interpretation of it was unreasonable given the wording.
7. (Original post by bobo19966)
The only difference between two or three marks is the mention of ANY distribution the three marks one is the perfect definition
Thank you
8. (Original post by Navo D.)
Are you showing the existence of some correlation? Then its two tail
Are you showing a positive or a negative correlation? Then its one tail
Ho - P=0
H1 - P>0

for PMCC, what are the hypothesis?
9. (Original post by tazza ma razza)
Ho - P=0
H1 - P>0

for PMCC, what are the hypothesis?
what do you mean lol
10. (Original post by kidder007)
Thank you
Sorry forget to mention population should be population of any distribution
11. (Original post by Navo D.)
what do you mean lol
dw i can't even remember i know what to do now, just spoke to my mate !
12. What are they usually looking for in the assumption questions, in general?
13. Prediction's anyone?
14. (Original post by nayilgervinho)
Prediction's anyone?
normal distribution
bias
definitions (s2 ones as well)
differences in mean
spearmans rank
contigency table
co variance
+ something esle
15. Do we round expected frequencies to 2 dp or write them as they are?
16. i am going over bias estimators now and i wanna jump off a cliff
17. (Original post by nayilgervinho)
Do we round expected frequencies to 2 dp or write them as they are?
write them in full then round them to 4 dp. When you use them, use them rounded to 4 dp but give your final answer to 2 dp
18. (Original post by roscoe xbo)
June 10 Paper

Q3a) A woodwork teacher measures the width, w mm, of a board. The measured width, X mm,is normally distributed with mean w mm and standard deviation 0.5 mm. Find the probability that X is within 0.6 mm of w.

I don't understand why in the mark scheme they do: 2 x 0.8849 - 1 = 0.7698. Can someone explain please?

If you do that question correctly you should end up with P(|Z| <1.2) . This means P(-1.2<Z<1.2). If you draw out a normal distribution graph of that desired area you want, you'll find its P(Z<1.2) - P(Z<-1.2)

This is the same as P(Z<1.2) - (1- P(Z<1.2)) = 2P(Z<1.2) -1

Which is (2 x 0.8849) -1. Lol I was very confused when I first saw this question, but it has come up a few times now
19. (Original post by tazza ma razza)
normal distribution
bias
definitions (s2 ones as well)
differences in mean
spearmans rank
contigency table
co variance
+ something esle
What exactly do you mean by bias?
20. Solomon Paper C, Q3c, why is the answer 11.08 and not 10.48? Surely you take the 0.6 away as it is a BIASED estimator for k?

Edit: stupid mistake

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