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

1. Oh well, 9 or 10... got kinda confuzzled doing everything backwards. Method marks all round!
2. why was it not 10??? loadsa ppl got 9 or 10, and a few ppl even got 11? wot was the working for that question???
3. im sure it was postive skew as the median was greater then the mode and also the mean.
4. when n=20 and P=0.25
P(X<<10)=0.9961>0.99

therefore n=20-10=10...?
5. (Original post by riks2004)
im sure it was postive skew as the median was greater then the mode and also the mean.

na..........

Mode> median>mean

therefore negative skew
6. i got root 2 for median i factorised it so (m^2 - 2 )(m^2 - 2) = 0
7. why not 11 - no way it can be 10 i think, because you have to do the c-1 thing dont you? i.e. 9 if you didn't flip, and 11 if you did, i got something like y>=8 or something and then got 9 from that donig c-1=8, then i thought we had to flip again because you are interseted in the number of successes and not the number of untreated cases - well i did something like that...hopefully will get some marks - i need a high A in this mod cos i messed up my P5 on monday

got to revise for P4, P6, M4 next week(

phil
8. guys u need to specify if its x<9 or less or equal too.

as x<10 is the same as x is less or equal to 9.

i got x<9 or x is less or equal to 8.
9. (Original post by riks2004)
i got root 2 for median
no way that can be right - its def 1.08, cos you get 0.5 when you sub this into the cdf - i checked it
10. (Original post by tiantang)
guys u need to specify if its x<9 or less or equal too.

as x<10 is the same as x is less or equal to 9.

i got x<9 or x is less or equal to 8.

the question asked for the maximum value!
11. (Original post by tiantang)
guys u need to specify if its x<9 or less or equal too.

as x<10 is the same as x is less or equal to 9.

i got x<9 or x is less or equal to 8.
was a very cnofusign two questions huh? i did the rest of the paper in 0.5 hours, and spent an hour thinking about that one - and still think i got it wrong
12. (Original post by Phil23)
no way that can be right - its def 1.08, cos you get 0.5 when you sub this into the cdf - i checked it

i disagree im sure it was root 2, any1 got the paper so we can check
13. (Original post by Savioahang)
the question asked for the maximum value!
right hope i dont marks knocked off - i only gave critical region - hope markers are not stingy about minor details
14. 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
15. (Original post by Phil23)
no way that can be right - its def 1.08, cos you get 0.5 when you sub this into the cdf - i checked it
i got √3 for one of them think it was the mode
16. (Original post by marisad_uk)
i got √3 for one of them think it was the mode
i got rt (4/3) for that
17. (Original post by tiantang)
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.
from tables i got c-1=9 therefore c=10 and i just left it at x=10 for answer
18. (Original post by Phil23)
i got rt (4/3) for that

same
19. (Original post by marisad_uk)
from tables i got c-1=9 therefore c=10 and i just left it at x=10 for answer
if you did greater than 99% (which i believe is correct than it is c-1=10. I have just checked the tables at home.
20. YESSS! I got 9 for the last answer as well. And negative skew. Woooo! Oh and route 4/3.

Best exam I've done so far hopefully it will make up for the disaster that was P3.

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