# June 2018 Stats

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#1

Hi, just doing the 2018 paper, and the pure section was fine, but I’ve completely flopped this stats paper, which is a shame, as Stats is usually the thing I do the best in. Literally had no clue for Question 1 - but in the markscheme it has C from 0 to 8 (and then a probability of 1/9 for each), but I’m just really confused where u even get those numbers from, as there’s literally nothing in the Question ?
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4 weeks ago
#2
In the large data set (Edexcel), cloud cover is measured in oktas, integer values from 0 to 8, hence the range 0 to 8. Each of these values has an equal probability, so the probability of each value is 1/9. The large data set is often neglected by students, so I'd imagine a lot of other students in 2018 were in the same situation as you. I hope that this helped.
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#3
Ah, thanks a lot man. I’ve tried a lot trying to memorise the large data set - just doesn’t come natural to me, so yeah I messed up that question. It was really weird because I think I got 43/44 on the 2019 paper and all the other papers I did, I was averaging around 40-45 and then this paper I got 20s. So idk what to make of it
Last edited by Madman11; 4 weeks ago
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#4

I’m having lots of trouble with this question as well ngl. Parts A and D are fine, but I’m just not grasping B and D atm.
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4 weeks ago
#5
For b) how do you represent the 4 batteries working and the extra 4 hours compared to a)? Note the question asks about the remaining exam, so really you have to condition on the first 16 hours being ok.
For c) you'd have to model two batteries working for 20 hrs and two for 16 and 4 hours.
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#6
I’ve managed to get the answer .... but a conditional probability in a normal distribution question - genuinely never seen that. Also, I was a bit confused on why we’d find out the probability of X>20 and X>16, instead of X=20 and X=16. I know In question 1 it asks for the probability that X>16, so I’m just wondering whether it’s to do with that ? Because when you’re actually reading the question, it says she has used her calc for 16 hours, not more, and then it claims she’s got another 4 hours, which would take us to bang on 20 hours, so not really sure why we assume > and use the cumulative distribution instead of assuming = and using the PD.
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4 weeks ago
#7
I’ve managed to get the answer .... but a conditional probability in a normal distribution question - genuinely never seen that. Also, I was a bit confused on why we’d find out the probability of X>20 and X>16, instead of X=20 and X=16. I know In question 1 it asks for the probability that X>16, so I’m just wondering whether it’s to do with that ? Because when you’re actually reading the question, it says she has used her calc for 16 hours, not more, and then it claims she’s got another 4 hours, which would take us to bang on 20 hours, so not really sure why we assume > and use the cumulative distribution instead of assuming = and using the PD.
It is a CDF questiion, as many questions involving the normal distribution (pdf) are at a level.
You want to see if the lifetime is > 16 (exam 1) or > 20 (exam 2). These are cdf range questions as the batteries will eventually fail (probability 1). The cdf goes from 0->1 as X increases. The actual value of the pdf tells us about the relative (density) probability with which these things occur, but you want to find the area under the density curve, i.e. the cdf.

You've obviously worked out how split the 0->20 into 0->16 and 16->20 and use the joint / conditional to get the exam 2 info the question asked. Let me know if you're still unsure (maybe post what you've done)?
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#8
So should I always use the CDF function, unless the question specifically asks ‘X=5’ or something like that ? Because actually now that we’re talking about this, I don’t think I’ve ever used the Normal PD function on my calculator ? Is that not on the Maths spec or something
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4 weeks ago
#9
So should I always use the CDF function, unless the question specifically asks ‘X=5’ or something like that ? Because actually now that we’re talking about this, I don’t think I’ve ever used the Normal PD function on my calculator ? Is that not on the Maths spec or something
It's a lot more usual to use the cdf and talk about ranges than it is is evaluate the pdf for a specific value.

Note, in a normal distribution, the probability that X=5 (for instance) is basically zero. It's a density function on the real axis, so the probability that the event has exactly that value is zero. However, it makes sense to talk about ranges, but that's a CDF (area under the density curve).
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#10
By the way, what do you reckon the likelihood is that the stats and mechanics paper in a couple of weeks is gonna be as tough as the 2018 one ? I found the 2019 one very straightforward for the most part, especially compared to 2018, so I’m just wondering if it’s likely to be extra tough this year, considering how easy it was last year.
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4 weeks ago
#11
By the way, what do you reckon the likelihood is that the stats and mechanics paper in a couple of weeks is gonna be as tough as the 2018 one ? I found the 2019 one very straightforward for the most part, especially compared to 2018, so I’m just wondering if it’s likely to be extra tough this year, considering how easy it was last year.
It's not my area, so can't really say - sorry. I take it you're doing the "resits"?
If there are not too many examples of your exam board, maybe do the other boards as well to get some robustness about the type of questions that may be asked.
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#12
Yeah mate, I’m on for the October resits. Thanks anyway man, you’ve been a big help 1
4 weeks ago
#13
(Original post by mqb2766)
It's a lot more usual to use the cdf and talk about ranges than it is is evaluate the pdf for a specific value.

Note, in a normal distribution, the probability that X=5 (for instance) is basically zero. It's a density function on the real axis, so the probability that the event has exactly that value is zero. However, it makes sense to talk about ranges, but that's a CDF (area under the density curve).
I thought when they ask for x=5 for instance you use normal pd, as x =5.
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4 weeks ago
#14
(Original post by Ferrari08)
I thought when they ask for x=5 for instance you use normal pd, as x =5.
For a discrete probability (mass) function, then yes.

For a continuous probability density function (like a normal distriution) the probability that the variable takes an exact value, say X=5.3453... is zero. The probability density function tells you what the probability of a range is, say 5<X<5.5, by finding the area (probability) under the pdf curve for that interval.
https://en.m.wikipedia.org/wiki/Prob..._mass_function
https://en.m.wikipedia.org/wiki/Prob...nsity_function
That's why you use the cdf a lot.
Last edited by mqb2766; 4 weeks ago
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