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# S2 ocr mei 2016

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1. Hey everyone! How was it?!

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2. Okay, what did you get for your rank correlation?
3. For the first question, (scatter graph) what did people use as the vertical and horizontal axis? I used percentage as the y axis because it seemed logical to me but I'm not so sure now!

What is the assumption needed for spearman's rank. Some people said they put 'no assumption' as the answer but I just guessed at bivariate normal distribution because the PMCC is similar...

I screwed up 3iv really badly with the simultaneous equations to find the mean and SD, but luckily I had time to redo it in the extra space and got 46040 for the mean... If I remember correctly, and 5509 for the SD? (To 4sf)

Was quite happy with all of the hyp tests, can't remember what results I got though, although for the last one I did a silly and calculated the SD as npq which came out as 52.8 I think... I got a sensible answer though so I'm not sure if that's right or not.

Hope this helps!
4. (Original post by Osmium)
Okay, what did you get for your rank correlation?
I think I got -0.664, I got 366 for Ed^2
5. I have no idea why I was getting the postive of that value after two attempts. Could be because I was ranking in reverese.
6. (Original post by Osmium)
I have no idea why I was getting the postive of that value after two attempts. Could be because I was ranking in reverese.
I think you might have done
(6xEd^2)/n(n^2-1) -1
when it is supposed to be
1- (6xEd^2)/n(n^2-1)
That would give you 0.664, but don't worry, you will still get marks for ranking, getting Ed^2 and using the main part of the formula!

Ranking in reverse is still right as long as you use the same method (low-high or hig-low) for both variables. Because the values are then squared so it doesn't matter if they are pos or neg.
7. I forgot to square the mean for final hypothesis testing when calculating the variance.
Messed up the standard deviation, mean + distribution diagrams in qs 3 (right method, wrong values and therefore wrong diagram).
Messed up a Normal calc worth 3 marks (method marks maybe?)
Did not draw the scatter diagram in 1 but labelled the axes.
Used Normal to the Poisson instead of to the Binomial.

What grade do you think I'd get? I know it's entirely dependent on the grade boundaries but based on what you think they might be, I'd like an idea of how badly I've done.
8. (Original post by 89911998a)
I forgot to square the mean for final hypothesis testing when calculating the variance.
Messed up the standard deviation, mean + distribution diagrams in qs 3 (right method, wrong values and therefore wrong diagram).
Messed up a Normal calc worth 3 marks (method marks maybe?)
Did not draw the scatter diagram in 1 but labelled the axes.
Used Normal to the Poisson instead of to the Binomial.

What grade do you think I'd get? I know it's entirely dependent on the grade boundaries but based on what you think they might be, I'd like an idea of how badly I've done.
You'd probably lose 2-3 marks maximum for forgetting to square the mean.

For the standard deviation, mean and distribution diagram question I'm pretty sure they will give you method marks if you are incorrect, and they would probably mark the diagram using your current value and see if it matches up. It would be pretty horrible if they instantly gave you no marks for not using values that you were meant to get. All in all, about 2 marks as well? (Assuming that your diagram is good and drawn according to the standard deviation and mean YOU got.

Depends on what the normal calculation was, how did you mess up?

By labelled the axes do you just mean put "% population" and "fertiliser use" in the graph? If so you lost 2 marks I think.

Which question was this? Or do you mean approximating to the Normal Distribution via Poisson or Binomial?

All in all, even I don't know what grade you'll get lol, I made stupid mistakes this paper too which might bump me out of a standard A grade on this paper. I already have 5 marks I know I have lost for stupid reasons + I also don't completely know how the diagram question will work since like you I calculated sigma and the mean wrong, but with the right method.
9. hey guys,

i accepted Ho on all of the tests, what about you?
i also plotted the %on the x axis and fertiliser on the y axis , is that wrong?
10. (Original post by Yasmine Grizel)
hey guys,

i accepted Ho on all of the tests, what about you?
i also plotted the %on the x axis and fertiliser on the y axis , is that wrong?
i remember for the last question i got a significant result of the probability for the mean to be greater than 373/60, to be 0.52% therefore significant

variance was sxx/n-1,(n-1 being 59) and variance for the hypothesis test was Sxx/((n-1)*60)
11. I rejected the null hypothesis for the last question bang!

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12. for question 3iv) for mean i got 50350 (or something close) and for sd i got about 6000 something can't remember the exact values of either but both mean and sd were higher than the given distribution mean and sd, did anyone have similar answer as me? :/
13. (Original post by Tajwar786)
for question 3iv) for mean i got 50350 (or something close) and for sd i got about 6000 something can't remember the exact values of either but both mean and sd were higher than the given distribution mean and sd, did anyone have similar answer as me? :/
i remember getting in the 50000 can't remember what it was exactly though
14. Anyone know how the sketching normal distributions question will be marked? I labelled the horizontal axis with the means, and made sure the distribution with the larger variance was shorter and fatter, but apart from that I didn't know what to do...
15. (Original post by Ting07251997)
i remember for the last question i got a significant result of the probability for the mean to be greater than 373/60, to be 0.52% therefore significant

variance was sxx/n-1,(n-1 being 59) and variance for the hypothesis test was Sxx/((n-1)*60)
ohh, i worked out the variance differently i was confused bc we dont usually get questions like that, they just give you the variance which is much nicer , its so annoying bc i know how to do the test perfectly and now i got it wrong bc of that . do you think i can pick up some method marks?
16. definitely for the H0 and H1, but not sure whether they will give you the calculation method marks because your variance doesn't seem like it was correct meaning that unless there is error carry forward mark which there some time is then you would probably get max 6 marks, otherwise 3 for H0 and H1 and if you have defining mull as the population mean of the given context of the question
17. (Original post by Yasmine Grizel)
hey guys,

i accepted Ho on all of the tests, what about you?
i also plotted the %on the x axis and fertiliser on the y axis , is that wrong?
I rejected h0 for spearmans test, accepted h0 for chi squared test and rejected h0 for the 11 marker
18. This is what I can remember

Q1catter diagram:
1 mark for labelled axes, 2 marks for correctly plotted points
Spearmans: -0.366
Spearmans hypothesis test: accept H0.
Pearsons not appropriate since not bivariate normal because not elliptical.
No assumptions required for Spearmans.
1% significance level is 1% probability of rejecting H0 when it is in fact true.

Q2:
Independent: mutations dont affect probability of another mutation, random:mutations are unpredictable
Binomial calculation: 20C1 x 0.012^1 x 0.988^19 or something like that
Approximation to posission: mean = 500x0.012, then use tables etc.
Approximation to normal, mean=50000x0.012, variance=50000x0.012x0.998 then standardise and use tables etc.

Q3:
Simple normal calculations
Manufacturer claims over 95% etc.: calculate value= 0.9502 so claim is true.
Find sigma and mu, sigma =6350 and mu = 60500 or 50850 or something like that?
Draw distributions: X has lower mean and variance so less spread but taller
Calculate value of h when 99.9%: I got 40500 or there abouts

Q4:
Calculate expected frequency of female and likes maths, show contribution=...
Chi test, add up contributions, accept H0.
Normal test, calculate mean and variance or standard distribution, calculate test statistic, reject H0.

If any one remembers anything else let me know.
19. (Original post by ROSS1998JONES2)
This is what I can remember

Q1catter diagram:
1 mark for labelled axes, 2 marks for correctly plotted points
Spearmans: -0.366
Spearmans hypothesis test: accept H0.
Pearsons not appropriate since not bivariate normal because not elliptical.
No assumptions required for Spearmans.
1% significance level is 1% probability of rejecting H0 when it is in fact true.

Q2:
Independent: mutations dont affect probability of another mutation, random:mutations are unpredictable
Binomial calculation: 20C1 x 0.012^1 x 0.988^19 or something like that
Approximation to posission: mean = 500x0.012, then use tables etc.
Approximation to normal, mean=50000x0.012, variance=50000x0.012x0.998 then standardise and use tables etc.

Q3:
Simple normal calculations
Manufacturer claims over 95% etc.: calculate value= 0.9502 so claim is true.
Find sigma and mu, sigma =6350 and mu = 60500 or 50850 or something like that?
Draw distributions: X has lower mean and variance so less spread but taller
Calculate value of h when 99.9%: I got 40500 or there abouts

Q4:
Calculate expected frequency of female and likes maths, show contribution=...
Chi test, add up contributions, accept H0.
Normal test, calculate mean and variance or standard distribution, calculate test statistic, reject H0.

If any one remembers anything else let me know.
Just as a note to other people, you would also get the marks if you approximated to normal using Poisson distribution, it would just be X~N(600,600)
20. (Original post by Studious_Student)
Just as a note to other people, you would also get the marks if you approximated to normal using Poisson distribution, it would just be X~N(600,600)
What use 2 approximations, from Binomial to Poission and then Poission to Normal? Seems unnecessary for me and I'm not pretty sure they wont credit that on the mark schemes, never seen them credit that before. Unless you can give me an example otherwise?

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