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OCR MEI S3 20th January 2012

Tharshan1992

anyone doing this exam?

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Reply 1

Rajbaj

Yeah and crapping myself. How r u finding revision?

Reply 2

Tharshan1992

am just doing past papers and past papers best way of learning S3 and yet finding it hard man :frown:

hope tomorrow paper is easy

Reply 3

Naum

Original post by Rajbaj

Yeah and crapping myself. How r u finding revision?

good luck hussein :tongue:

Reply 4

Beth1234

Now is possibly not the best time to realise that you hardly know the hypothesis tests...

I'm doomed. :eek:

Reply 5

Beth1234

How did everyone find it...?

Reply 6

Tharshan1992

couldnt answer question 4 :frown:

other than that it was a decent paper

Reply 7

Beth1234

Original post by Tharshan1992

couldnt answer question 4 :frown:

other than that it was a decent paper

Yeah, question 4 I found a little tricky... do you remember how you did the last part?

Reply 8

Tharshan1992

the way i believe your suppose to do it is like x +/- 2(sd) and then compare it with the value they gave which I wrote as doubt. however, not too sure since my value for E(X) was 100% incorrect :frown:

I was pretty annoyed after how hard c3 was and couldnt focus at all in s3 :frown:

Reply 9

Beth1234

Original post by Tharshan1992

I was pretty annoyed after how hard c3 was and couldnt focus at all in s3 :frown:

What makes you think it was 100% incorrect? My E(X) value seemed quite far from the given population mean given...

I did the last part using confidence intervals at the 90% significance level, and commented that as 90% of confidence intervals should include the mean, and that my confidence interval didn't, the model couldn't be very good (obviously phrased a little differently!).

Reply 10

Tharshan1992

hmm that brought my hopes abit up, lol

how did you find question 2, I found it so confusing

Reply 11

Beth1234

Original post by Tharshan1992

hmm that brought my hopes abit up, lol

how did you find question 2, I found it so confusing

Ah, the one where referring to the Normal tables was driving me mad...

If I remember correctly, the question can in a nutshell be written like this.
Small packets have distribution S~N(505, 11^2)
Large packets have distribution L~N(1005, 17^2)
Then just used various combinations of them :s-smilie:

So for the part that said "Find the probability that two randomly chosen small packets with have a larger* weight that a randomly chosen large pack", that's the same as finding P(S1+S2>L) So I found the distribution of S1+S2-L and carried on from there. I hope some of that was right!

(*It might have been "smaller", I really can't remember.)

Did you do interpolation on the Normal tables? I can't remember if you're meant to...

Reply 12

Rajbaj

I hated that normal question sooo annoying and i had a complete blank for a while. ended up doing some messed up stuff with z=25 over S.D in the end. and that was for earlier parts!!!

Reply 13

Rajbaj

And yeah I'm pretty sure you have to interpolate... :s-smilie:

Reply 14

Beth1234

Original post by Rajbaj

And yeah I'm pretty sure you have to interpolate... :s-smilie:

Yeah, I did interpolate - it took a little while...

Did anyone else find timing a bit of an issue with this paper or was it just me?

Reply 15

Joe Streets

I really thought that Q4 part one was an awful question

Had no idea where to start with that although thankfully the rest of the Q wasn't dependant upon that so I think I'm fine there

Reply 16

Beth1234

Original post by Joe Streets

I really thought that Q4 part one was an awful question

Had no idea where to start with that although thankfully the rest of the Q wasn't dependant upon that so I think I'm fine there

I think this is how you work it out:

The question stated that the probability that the dart landed on the dartboard (in other words, the probability that R<r) is proportional to the area of the dartboard.

So we can say

P(R<r)=k\pi r^2

where k is a constant. (1)

Now we know that if r=a, the P(R<r)=1. (a was the radius of the circle that the dart lands in, I think, so if the dart lands in a circle of radius a=r, it must land on the dartboard, so P(R<r)=1)

Substituting, we get

1=k\pi a^2

k=\frac{1}{\pi a^2}

Now substituting the value of k back into (1), we see that

P(R<r)=\frac{r^2}{a^2}

Now it would have helped if I actually thought of that in the exam... :redface:

Reply 17

Joe Streets

I think the question wasn't phrased as best it could be.
So annoyed that I didn't see that!

Reply 18

OllieReynolds

Did everyone find the CV for the confidence interval in 2.(v) by using the t-distribution or the Normal distribution tables? Because n=20 (which is small, so i thought t-distribution would be correct), but there isn't a value for v=19 in the t-distribution table... :s-smilie:

Reply 19

B Jack

The normal distribution question where you had to find the probablity that two small bags differed by less than 25 grams, were you meant to find P(-25<S1-S2<25)? I don't remember seeing a question along these lines in any of the past papers so it confused me a little :s-smilie:

Generally though I thought the paper was ok, anyone remember any of the test statistics they got for any of the questions?