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Edexcel S3 - Wednesday 25th May AM 2016

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Avatar for Rkai01
Rkai01
6
Original post by paradoxequation
Has anyone done this question? In part a, you get 2.54 as an unbiased estimate for the variance, but then it straight up says that the variance is 2.56 despite this information only coming up in part b which hasn't happened yet.

Also, in part b why do we use 2.56 and not 2.54? Thanks.


DW about that they make hella printing errors.

For part b we are given the official sd which we always choose over an estimated value
Original post by paradoxequation
Has anyone done this question? In part a, you get 2.54 as an unbiased estimate for the variance, but then it straight up says that the variance is 2.56 despite this information only coming up in part b which hasn't happened yet.

Also, in part b why do we use 2.56 and not 2.54? Thanks.


Edit: There's no error. s2s^{2} is the unbiased estimator for variance, not σ2\sigma^{2}
(edited 7 years ago)
Original post by paradoxequation
Has anyone done this question? In part a, you get 2.54 as an unbiased estimate for the variance, but then it straight up says that the variance is 2.56 despite this information only coming up in part b which hasn't happened yet.

Also, in part b why do we use 2.56 and not 2.54? Thanks.


I don't quite understand what you are saying there.

I suppose you could use a t test in part b but it has told you what the variance is.. so I guess that is the indicator that they want you to use the normal distribution.
Reply 603
Avatar for Rkai01
Rkai01
6
Original post by Euclidean
Edit: There's no error. s2s^{2} is the unbiased estimator for variance, not σ2\sigma^{2}


No there is they wrote 2.56 above the letter b which makes it look like an answer for part a when they actually write it for use in part b. I've seen this kinda thing before by them
Original post by Rkai01
No there is they wrote 2.56 above the letter b which makes it look like an answer for part a when they actually write it for use in part b. I've seen this kinda thing before by them



It's just badly presented. The σ2\sigma^{2} is supposed to be next to (b) like you say. But the answer is there in the red box :tongue:
WOOOOOOWWWW the june 15 ial paper was a pile of piss in comparison to the june 15 normal paper what is up with that?!?!
Original post by tazza ma razza
WOOOOOOWWWW the june 15 ial paper was a pile of piss in comparison to the june 15 normal paper what is up with that?!?!


On that note, which past papers are most difficult for S3?

I need to plan which ones I'll do in timed conditions as mocks, June 2015 for certain :redface:

Tagging @Zacken because he has a very good memory :tongue:
do less degrees of freedom imply that the model is improved?
textbook says 'The continuous uniform distribution only has the requirement that the expected frequencies must equal the observed frequencies. Thus v = number of cells -1.'

does this mean that u dont have to combine expected frequencies that are less than 5?
Original post by physicsmaths
do less degrees of freedom imply that the model is improved?


uhhh yh i think so - check the graph of chi squared
Original post by connorbarr
textbook says 'The continuous uniform distribution only has the requirement that the expected frequencies must equal the observed frequencies. Thus v = number of cells -1.'

does this mean that u dont have to combine expected frequencies that are less than 5?


You would, then the nmber of cells would decrease accordingly (then take away one at the end).
Original post by SeanFM
You would, then the nmber of cells would decrease accordingly (then take away one at the end).


doesnt say anything about combining expected frequencies was why i asked, so should i always combine Ei < 5 regardless of the distribution?
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Avatar for Rkai01
Rkai01
6
Original post by tazza ma razza
WOOOOOOWWWW the june 15 ial paper was a pile of piss in comparison to the june 15 normal paper what is up with that?!?!


What you have done if q 4a actually asked "Find the probability that THREE randomly chosen sacks differ in weight by more than 0.5kg" e.g one weights 1kg another 1.6 and 2.3, how would you set the question up?
(edited 7 years ago)
Original post by connorbarr
doesnt say anything about combining expected frequencies was why i asked, so should i always combine Ei < 5 regardless of the distribution?


I would say so. It's unlikely that you would need to for the uniform distribution (there is a past paper question somewhere with a unif distribution) unless the widths are disproportionate - eg if you thought something was ~U(0,10) and one class was 0-0.5 and the next one was 0.5-3.
Original post by Rkai01
What you have done if q 4a actually asked "Find the probability that THREE randomly chosen sacks differ in weight by more than 0.5kg" e.g one weights 1kg another 1.6 and 2.3, how would you set the question up?


hmmm

i reckon i would @Zacken for help haha
Original post by SeanFM
I would say so. It's unlikely that you would need to for the uniform distribution (there is a past paper question somewhere with a unif distribution) unless the widths are disproportionate - eg if you thought something was ~U(0,10) and one class was 0-0.5 and the next one was 0.5-3.


sound thanks
Original post by Rkai01
What you have done if q 4a actually asked "Find the probability that THREE randomly chosen sacks differ in weight by more than 0.5kg" e.g one weights 1kg another 1.6 and 2.3, how would you set the question up?


I think that would just be a can of worms.

The question is slightly ambiguous as it is - what does it mean for THREE things to differ by more than some weight?
Original post by Euclidean
On that note, which past papers are most difficult for S3?

I need to plan which ones I'll do in timed conditions as mocks, June 2015 for certain :redface:

Tagging @Zacken because he has a very good memory :tongue:


June 13R is a bit weirder but tbh they're all okay. Any hard solomon papers?
Original post by Nikhilm
June 13R is a bit weirder but tbh they're all okay. Any hard solomon papers?


They're all pretty standard

I think S3 is distributed relatively uniformly in terms of difficulty.

Feel free to test that hypothesis if you'd like! :biggrin:
Original post by tazza ma razza
let me know how you do - i am doing it atm


I just finished it and tbh it was the hardest s3 paper I've sat, I really struggled with the latter part of q5 but the rest was doable, how did you find it?

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