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Ocr (mei) s2 exam - 22/06/11

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Reply 140

IchiCC

For the regression question I left the value of the gradient as 7/120. Would that lose marks, and if so how many?

Reply 141

ramsandgold

I dont expect so. I first did it as 0.0583 but I went back and changed it. The differences in the answers are quite big.

Reply 142

RacingPro97

Original post by Evangelica

I got that but I think we may be wrong.

Looking at the exact distribution in Q2 again:

The exact distribution cannot be Normal, as the data is not continuous - you cannot have a 1/3 of a car, for example. It cannot be Binomial either, as there was no value for 'p' - 'p' cannot equal 0.62, as if the mean was above 1 then we would have a 'p' above 1, which is impossible. So surely it should be Poisson?

Reply 143

Evangelica

Original post by RacingPro97

I taught myself most of S2 on Tuesday and have already forgotten a lot, but that's what I thought. It just seems like the majority went with a Normal distribution. Well I hope we're right; I need an A to get into uni and I'm pretty sure S2 is borderline A/B!

Reply 144

Gaige

Correct me if I'm wrong but can't you approximate poisson using normal and ex=var. Well I didn't use this but surely because the mean and the variance of the binomial were very close surely it wouldnt be too far out using a Normal approximation in that case.
Just a thought for those that used normal

Reply 145

ILM16

I used a Poisson (37.2) for Q2, as did many others I know.

Reply 146

hhello

Paper is attached , could someone kindly write up a mark scheme for it. Thanks

4767_June11.pdf57.3KB

Reply 147

karchun

Original post by hhello

Paper is attached , could someone kindly write up a mark scheme for it. Thanks

Wow how did u guys get this info...LOL thats bare leakage!!

Reply 148

hhello

Original post by karchun

Wow how did u guys get this info...LOL thats bare leakage!!

What do you mean, my maths teachers email us the papers once they come out. They mark for OCR so they have access to these around 2 days after the exam is sat..

Reply 149

huynhster

So for question 3) iii) B)

Use a suitable approximating distribution to find the probability that in at most one of these pack all 3 apples weigh at least 220 grams.

I used the modelled using the normal distribution, when I ask the teacher he said it was perfectly alright to use poisson and normal, and it sort of made sense using the normal aswell. what do guys think??

my answer 0.3897

AND 3) iii) C) Explain why this approximating distribution is suitable.

I put its because the mean is reasonably close to the variance.

X~B(100,0.0188) ===>> np= 100 x 0.0188 = 1.88
npq= 100 x 0.0188 x (1-0.0188) = 1.84

X~N(1.88, 1.84)

Reply 150

SnookerLoopy

^^

3) iii) C) Explain why this approximating distribution is suitable

I put because N is large and P is small??

Reply 151

karchun

Original post by SnookerLoopy

^^

3) iii) C) Explain why this approximating distribution is suitable

I put because N is large and P is small??

Same here but i didn't do the top bits correct - out of a random guess - i was contemplating if i was suppose to add np is not close to 0 or 1 but 0.0188 is very close !!

Reply 152

Gaige

Original post by ramsandgold

I dont expect so. I first did it as 0.0583 but I went back and changed it. The differences in the answers are quite big.

I put the answer for when 0.0583 and for when it's 0.05833333333333
Will they see that the appropriate answer I'm trying to get at and just give it me because rounding errors give 2 answers and I did write this next to both answers

(edited 12 years ago)

Reply 153

aaarie

Original post by Joe D

That's what I put, although I was doubting myself because the question was worth 2 marks. Could they give one mark for 'Poisson' and one mark for the number? Seems generous if that's the case.

It certainly wasn't a binomial distribution. It could've only been that if they'd wanted the number of minutes in one hour in which a certain number of cars arrived, but I seem to remember it was asking for the distribution just for the number of cars arriving in one hour.

If they ask you for the EXACT distribution of the probability in S2 it is always binomial because the poisson and normal are both approximating distributions and not exact.

(edited 12 years ago)

Reply 154

RacingPro97

Original post by aaarie

If they ask you for the EXACT distribution of the probability in S2 it is always binomial because the poisson and normal are both approximating distributions and not exact.

Not necessarily - all 3 distributions are exact for their particular sets of data. Yes, the Poisson and Normal distributions can approximate a Binomial, but a Normal can also approximate a Poisson by N(Mean, Mean). So in this case the Poisson distribution can most definitely be an exact distribution, followed by a Normal approximation to the Poisson in the last part of the question. It never says in a typical Poisson question that the answer is only approximate - for this paper, the second part of Q2 says "Find the probability of at most 1 car arriving in a period of 1 minute.", not "Find the approximate probability of at most 1 car arriving in a period of 1 minute.".