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# June 22nd 2005 - The Official S2 Discussion Thread! watch

1. for the last question, part c I think, I approximated to the normal distribution instead of reversing it, which I did in the last part of the question. will I get 0/6? that would be sad
2. If you got the working out wrong, but put the hypothesis and also, despite the wrong working, got the bit about not rejecting H0 right...how many marks would you get for the 6 mark question?
Let's hope at least 4! No idea really. I hate thinking you did an ok paper and coming out and finding you did tons of things wrong. So annoying! I really needed a good grade
3. i got 0.275 or something for that...
4. 64/75 for an A? That seems obscene, because then getting 80% on the exam gives you less than 80% overall.

I know this isn't how UMS works out, but still, THAT harsh a scaling down seems unfair.

BTW, I got 9 people for the last question. I wouldn't want to be a marker this year, there is going to be quite a mess on some of the question 7s I feel, with people not defining terms properly etc.

I got an answer of 10.18% for the penultimate part of the last question. I still reject the null hypothesis, but I have no idea how I got this answer now!

Martin
5. I got 10 for the last part, but spent about 20 minutes deciding "Is it 9 or 10" ..the inequalities got really confuddling...however the rest of the paper was fairly easy, questions 1-5 were absolutely gorgeous, and whilst question 6 took some time with calculating things (especially the median!), it was relatively straight forward.

I think the fact there is huge disagreement about the correct answer for the last part of question 7 shows it was a nasty critter.

The mark for an A in Jan 2005 was something like 56/75, I expect this one to be around 60, as without the last question, it would have been 63 or so o__0
6. (Original post by tiantang)
Last question:

For successful recovery less than 75% , not recovered has to be greater than 25%.

x-Bi(20, 0.25)

Therefore (x 'greater or equal to' c) is < 1%
(x 'smaller or equal to c-1')>99%

from tables c-1=10, c=11

therefore x 'greater or equal to' 11 or more people who DO NOT recover!

therefore for people who DO recover.

x 'smaller or equal to 9.

For the cdf question:

mean was 1/1/15
mode was root (4/3)- you had let 'x^2' equal 'y' to get a normal quadratic equation.

and the pdf was defiantely negatively skewed as mean<median<mode

totally agree to all of that
i thought that wasnt a bad paper at all..just a lil trick here n there..but overall was very doable

for the P(2.3<X<3.4) it was 0.275

n first question n=125, s.d.=√2.88 = 1.697
mm wat else was there?
yeah median was 1.08, mean 1.07, mode 1.15
mean<median<mode
so yeah it was negative skew

does anyone hav the paper? i cant seem to remember any more questions at all!
7. (Original post by jessica_h)
for the last question, part c I think, I approximated to the normal distribution instead of reversing it, which I did in the last part of the question. will I get 0/6? that would be sad
Someone at my school did that. I'm not sure really, becuase n wasn't very large and p wasn't very small, whichever way around you took p to be. In summary, I've no idea =)

Martin
8. (Original post by pumpkinstein)
64/75 for an A? That seems obscene, because then getting 80% on the exam gives you less than 80% overall.

I know this isn't how UMS works out, but still, THAT harsh a scaling down seems unfair.

BTW, I got 9 people for the last question. I wouldn't want to be a marker this year, there is going to be quite a mess on some of the question 7s I feel, with people not defining terms properly etc.

I got an answer of 10.18% for the penultimate part of the last question. I still reject the null hypothesis, but I have no idea how I got this answer now!

Martin
It would be harsh. But it's happened in the past (in other modules at least)...
9. (Original post by pumpkinstein)
Someone at my school did that. I'm not sure really, becuase n wasn't very large and p wasn't very small, whichever way around you took p to be. In summary, I've no idea =)

Martin
I just thought it would be ok because np>5 and so was n(1-p). oh well... I did start off switching the probabilities around as well, v annoying
10. i got 1.71 for the standard deviation
11. wahoo i got all these i think, i just messed up 1b! duh

got 9 for the last question as the greatest value for sig 1% seems to be the right answer, all that c-1 stuff really confused me

1.07 1.08 1.15 for mean median and mode

did trial and error in the end for the median which is fine i suppose!

rejected h1 in the hypothesis question at the end because it was about 10%

did anyone get 0.07 for one of the questions? it was a normal approximation i think??? please someone say they did??
12. Well...

To everyone: How on earth did you get the s.d. for Q 1)b as it explicitly said that the data and the s.d. relate to a DIFFERENT sample?

Otherwise it was ok, though I had some problems finding the median - it was either zero (which is not quite possible) or sqrt 8, which is out of range. Anyone?

For the last question, I rejected H1 as the probability I got was around about 25% which was way off...

I got 9 people at most needed for the last last question, so that it could be significant at 1%.

One of the questions was rather bizarre as I had to work with a Binomial with a probability of .75 (something I had rarely done before, all probabilities have been less than 0.5)...and then for the second part I used an approximation.

Overall I really liked it.. apart from 1) b and the judgment for the skewness, which I said it was negative..but took a shot in the dark.
13. This test was so unbelievably easy. Should be 100% unless I've made some glaringly stupid mistakes.
14. (Original post by Kiril)
judgment for the skewness, which I said it was negative..but took a shot in the dark.
thats right mean<median<mode
1.07<1.08<1.15
15. (Original post by Finn)
did anyone get 0.07 for one of the questions? it was a normal approximation i think??? please someone say they did??
Yeah, that rings a bell, for something...
16. anyone for 0.07 in the normal approximation then????
17. yep it was 0.0694 so yeh wud be 0.07
18. (Original post by Kiril)
To everyone: How on earth did you get the s.d. for Q 1)b as it explicitly said that the data and the s.d. relate to a DIFFERENT sample?
Work out n in a similar way to part one: n = 75

X~B(75,0.04)

σ² = np(1-p) = (75)(0.04)(0.96)
σ = 1.70
19. (Original post by mockel)
Yeah, that rings a bell, for something...
GREAT! anyone else, i need another person to say they did to make my hypthesis correct, just one person is below my significance level
20. (Original post by Finn)
anyone for 0.07 in the normal approximation then????
Yes. It was 0.0694 from the tables.

The skewness on that other question was negative.

Skewness = (Mean - Mode)/Standard Dev

As the standard deviation is √variance, it can never be negative, so the numerator decides whether the skewness is negative or positive.

Mean - Mode was just below 0 so it was negative skew.

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