Probabilities & Statistics 1 - OCR (not MEI) - jan 25th 2012 discussion

Hey guys, just thought id open this up for the exam on wednesday

has anyone come across any difficult questions in particular that they feel would be useful to know about on wednesday.

In addition, Q9iii) of jan 2010

Use the formula for the sum to infinity of a geometric series to show

P(R=S)=p/2-p

i found out how to do this but would like to know the main trick, how do i find what my 'a' is and what my 'r' is from the original :

Stoinfinity=a/1-r

r=q^2 and a=p^2 according to the mark scheme

thanks in advance

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Reply 1

HypErTwisT

erm i learnt this a few days ago so im not perfect with it. But if you have a recuring line like with q9 on jan 10 ( p^2+(q^2p^2)+(p^2q^4 ect))
the a is the first value in the sequence so here, it is p^2 (p squared). the r in the equation is the common value( i think its called that), this is just how much the sequence increases by each time, so here it is multiplied by q squared. from there its just re-arranging till you get the answer.
Remember number sequences from primary? a= first value and r= how much it increases by each time.

thats pretty much it i think. anyone else taking this exam?

Reply 2

Tukazoar

yeah, are you's retaking it? Hope to get a nice grade in this to make up for the shambles of C3.

Reply 3

HypErTwisT

no this is my first a level exam. im not a big fan of s1 but im feeling ok about it i think

Reply 4

Ree69

...what are the conidtions for a geometric distribution?

Reply 5

zaraxai

Andrea practises shots at goal.For each shot the probability of her scoring is (2/5).
Each shot is independent of other shots.
(i) find probability that she scores her first goal on her 5th shot.

Can anybody help? :frown:

Reply 6

zaraxai

Original post by Ree69

...what are the conidtions for a geometric distribution?

Independent
Failure and Success

Reply 7

Ree69

Original post by zaraxai

Independent
Failure and Success

Cheers

Reply 8

Tukazoar

Original post by zaraxai

Its geometric, (assuming she doesn't get better with practice) so X ~ Geo (2/5)

P(1st goal on 5th shot) = four misses, then a goal = (3/5)^4 x (2/5) = 0.052

Reply 9

ben1994

how did the exam go everyone? does anyone know where someone may have posted thier answers?

Reply 10

HypErTwisT

i think it could have been alot worse what about you? erm i havent seen any other threads about this exam so im not sure

Reply 11

ben1994

I didin't do that well, couldn't do the very last part of the last question and realised i got a question wrong: i said the spearmans rank would stay the same (0.75) but just realised that is wrong

Reply 12

Matthew692692

I thought it would stay the same...

Reply 13

Ree69

I put the spearmans rank would increase...

Reply 14

MoneyOverEverythin

Original post by Matthew692692

I thought it would stay the same...

wouldnt it increase? d and everything remains the same but n(n-1) increases..

Anyone get about 0.412 for the being more red flowers than blue?

what were peoples answers for the last questions? 8 and 9

was there only 2 ways of getting 28 (9,8,7,4) and (9,8,6,5) so it was something like 48/3024 ?

also did anyone get 5/14 for something towards the end (8 or 9)?

Reply 15

Jedicake

Original post by MoneyOverEverythin

I got the same for all of these. (48/3024 simplifies down to 1/63 - an easier way to do this was just 2/9C4.)

Calculating the Spearman's rank was 1/2. And I think the height of the bar on the histogram was 2/3cm?

Reply 16

ben1994

For the question where you had to match the Geo and Bio distributions to the graphs did anyone get V and Y respectively?

Reply 17

Jedicake

Original post by ben1994

For the question where you had to match the Geo and Bio distributions to the graphs did anyone get V and Y respectively?

Indeed.

Reply 18

makeawish

Original post by MoneyOverEverythin

I got 5/14 for something, can't really remember what, and I also got 0.5 for spearman's rank. The value of spearman's rank increases so it is greater than 0.75 because the judges rank the last participant equally, so 6 (sigma d squared) is the same, but as there is one more person n increases by one, this means that the fraction is smaller, 1- a smaller fraction will create a larger fraction