What is this??? like what topic(Original post by Ayman!)
X <k/2 means that X < k, so we can just write it as X < k/2.
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Edexcel S2  27th June 2016 AM
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 25062016 22:45

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 25062016 22:49
(Original post by SSD07)
What is this??? like what topic 
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 25062016 22:52
Hi there, im stuck with this question, where for part a)i & a)ii i can't figure out how to hack the alternative method (2), especially for a)ii where for X~B(10,0.4) shouldn't P(X<9) be equivalent to P(x<=1). Plus for part b im lost. thanks in advance
Last edited by yuveethini; 25062016 at 22:58. 
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 25062016 23:54
(Original post by taysc)
Does anyone have a good way/acronym of remembering skewness in terms of mode median and mean?
Im struggling to remember the pattern
Post rating:1 
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 26062016 00:43

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 26062016 00:52
(Original post by yuveethini)
Hi there, im stuck with this question, where for part a)i & a)ii i can't figure out how to hack the alternative method (2), especially for a)ii where for X~B(10,0.4) shouldn't P(X<9) be equivalent to P(x<=1). Plus for part b im lost. thanks in advance
Now, X = 6 is the same as saying Y = 4. If you read that in English, it's the same as saying getting 6 heads is the same as getting 4 tails. So the distributions might be different, but they're modelling the same outcome (6 heads and 4 tails) and the water example is doing the same thing.
For part aii, the RHS, you may have missed out how they're using X and Y instead of just X. Saying X (the number of customers who order water, and Y is the number who don't)  you say that the probability of strictly less than 9 customers ordering water is the same as the probability of strictly more than 1 customer ordering water, and the probability of that is 1  p(less than or equal to 1 customer not ordering water).Post rating:1 
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 26062016 00:54
(Original post by MattOGrady)
S2 June 2011. Need help with question 6b please. Can anyone tell me if/why my working is incorrect?
In the mark scheme they go with working from this approach, but is the opposite true too? I worked from:
My full working and question below: 
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 26062016 00:57
(Original post by SSD07)
What is this??? like what topic
X<10 doesn't imply that X<5, because X could be 6,7,8,9 so the first one would be true but the second not.
But X<5 implies that X<10  if the first one is true, the second one is, so that's why the intersection of the two is X<5.
Similarly, if you had Y> 7 and Y > 11  the second one implies the first, but not the other way round, so the intersection would be Y > 11.Post rating:1 
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 26062016 01:09
(Original post by SeanFM)
The statements are pretty much equivalent (I'm not 100% sure about the first inequality sign in your second line of latex, it may have to be strictly less than 31 but I'm not sure) so I suppose you could work like that, though it'd be a bit strange. 
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 26062016 01:10
https://57a324a1a586c5508d2813730734...%20Edexcel.pdf
can someone help me with question 8
why did they e^2x/15. like i dont get where the exponential came from but i get the 2x/15.
https://57a324a1a586c5508d2813730734...%20Edexcel.pdf 
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 26062016 01:11
(Original post by MattOGrady)
Thank you! It's a normal approximation so I don't think it matters whether it's a strict inequality or not. Doing it this way just means that I can work from tables straight away, without having to do 1  probability later on. I guess I should just stick with the mark scheme method. 
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 26062016 01:25
(Original post by SeanFM)
I see what you're saying, but both regions include the critical value (which is what matter, you're right with the rest and like 30.9999999 and all of that) and I'm not sure if, by definition, it should be inclusive if you're going to do it that way  maybe I am just making things up ) but fair enough  personally, if you got it right that way I'd give you the marks but.. better safe than sorry, I s'pose 
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 26062016 01:27
the grade boundaries are so high wtf

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 26062016 01:40
(Original post by Lilly1234567890)
can someone help me with question 8
why did they e^2x/15. like i dont get where the exponential came from but i get the 2x/15.
Where as a result of the question, this becomes:
Then just sub in 1.65 and 1.75 giving values either side of 0.8, therefore showing that x=1.7 as required.Post rating:1 
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 26062016 01:45
(Original post by MattOGrady)
The exponential comes from the Poisson Distribution probability function in the formula booklet:
Where as a result of the question, this becomes:
Then just sub in 1.65 and 1.75 giving values either side of 0.8, therefore showing that x=1.7 as required. 
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 26062016 07:56
Help with this question please
Posted from TSR Mobile 
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 26062016 08:35
i have always known that if our value is less than 0.05. ten there is sufficient evidence to reject H0. But that doesnt seem to be the case here in q5
https://57a324a1a586c5508d2813730734...%20Edexcel.pdf
is the mark scheme wrong
https://57a324a1a586c5508d2813730734...%20Edexcel.pdf 
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 26062016 08:56
(Original post by MattOGrady)
I don't have an acronym to help but for some reason drawing these graphs help. Mode is always the 'peak' or most dense part of the PDF so draw that in, then draw in the median and/or mean depending on it's value being greater or less than the mode. If the PDF you draw is more dense towards the vertical axis (i.e. the mode is closest to the vertical), it is positive skew, and viceversa. This helped me, maybe I just have an odd way of learning things.
Thank you 
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 26062016 09:00
(Original post by SeanFM)
Think of it like this  if you flipped a coin 10 times, and you wanted to model how many times you get a head, say X is the distribution of number of heads and X~B(n,p) then you can also measure it with another distribution, Y, the number of tails. Y~(n,1p). Why are the probabilities that way? Because if p is the probability of getting a heads, 1p is the probability of getting a tails.
Now, X = 6 is the same as saying Y = 4. If you read that in English, it's the same as saying getting 6 heads is the same as getting 4 tails. So the distributions might be different, but they're modelling the same outcome (6 heads and 4 tails) and the water example is doing the same thing.
For part aii, the RHS, you may have missed out how they're using X and Y instead of just X. Saying X (the number of customers who order water, and Y is the number who don't)  you say that the probability of strictly less than 9 customers ordering water is the same as the probability of strictly more than 1 customer ordering water, and the probability of that is 1  p(less than or equal to 1 customer not ordering water).
Also how do i go about doing part b, i just dont understand itLast edited by yuveethini; 26062016 at 09:02. 
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 26062016 09:03
(Original post by SeanFM)
Number of correct answers = X. number of incorrect answers = 20  X.
number of points from total correct answers = ...
number of points docked from total incorrect answers = ...
hence...
it's not asking for you to find that it's when you get 4 correct answers and 16 incorrect answers (which doesn't quite make sense, though I see what you are trying to do)
It's asking how many points you get when X answers are right. (implying that 20X are wrong).
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Updated: August 20, 2016
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