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# Edexcel S1 - 15th June, 2016 [Exam Discussion] watch

1. sxy, sxx, syy and r

If i add additional data, what happens to them?

I know that if i add more to x and leave y the same, sxx can decrease if the sum of the additional bit is less than the mean of x (vice versa for increase) but what happens to r and sxy ?
2. (Original post by pranto96)
In June 2015 1(d) I am getting Q1 as 62! I don't know whats wrong! how do you get 9/15? My working is : 60+(21-15/21-6)*5. How it is possible to get 9 if u subtract 21-15??
Hey I think you might be a tiny bit confused about the formula for Interpolation.
21- 15 gives you the Cumulative Value - Group Value, and this gives you the value of the Previous Group. However, we're looking for how far the Lower Quartile is into the Group. So instead, you take 15 (The Median Value) - 6 (The Cumulative Total before that Group) to give you how far the Median is into the group. That gives you 9/15.

Hope that makes sense
3. (Original post by Florent venhari)
Hey Guys! Quick question! When writing the class width for any continuous data, do I always use the class boundaries when calculating the width???? For example, in question 5 in the JUNE 2014 R paper, the class width of the boundary 2-4 minutes is 3, but I put it as 2 :P
You always use the Class Boundaries, but remember to always look at whether the Class Boundaries are "Rounded" or "Exact Values". So for example, if they use Values where Classes "share numbers like this:

Then the Class Width is just 5. BUT if the Classes look like this...

Then you can see that there is rounding going on. So actually the Class Width is not 41-45, but 40.5-45.5. Which is a Class Width of 5.
4. s1 solomon i qn 4

https://0120b046f61e7ee5f95eb1315051...%20Edexcel.pdf

i swear this is wrong in the markscheme

https://0120b046f61e7ee5f95eb1315051...%20Edexcel.pdf
5. (Original post by hogree)
You always use the Class Boundaries, but remember to always look at whether the Class Boundaries are "Rounded" or "Exact Values". So for example, if they use Values where Classes "share numbers like this:

Then the Class Width is just 5. BUT if the Classes look like this...

Then you can see that there is rounding going on. So actually the Class Width is not 41-45, but 40.5-45.5. Which is a Class Width of 5.
Thank you soooo much!!!!!!!!!! Great explanation
6. In a set of discrete data where there are 12 values, why is it that when I look for Q1 and Q3 that it is the 3.5th value and the 9.5th value instead of the 3.25th/9.75th ? As in, why can't I look for the vaue a quarter of the way between the third and fourth?
7. Can anyone clear this up -
Do we ever have to use Interpolation with Normal Distribution? Or are we always allowed to round to the nearest value? (Say, we're given 1.5833 and not 1.58 or 1.59).
8. Do we need to know what a box and whisker plot is? Or just what a box plot is?
9. What's the difference between the frequency and the frequency density, and in what situation do we use the frequency density?
10. (Original post by Siddhart1998)
Do we need to know what a box and whisker plot is? Or just what a box plot is?
wtf - the exam is tomorrow how do you not know this....
11. After reading mark schemes I'm still not fully sure of how they mark the tests? it says to ignore any incorrect statements following a correct answer.
so for normal distribution questions where, for example, the correct answer was 0.9505, and i wrote that down, but afterwards i wrote 1-0.9505 and gave an answer of 0.0495 underneath. Would they ignore the last part or would I lose the mark?
12. (Original post by amelienine)
What's the difference between the frequency and the frequency density, and in what situation do we use the frequency density?
frequency density is frequency/class width and it is used in histograms. You will probably need to know it especially in questions where they ask you what the width and height of a bar will be (although they don't tell you that's frequency density) and drawing histograms.
13. This is gonna sound dumb but:

If P(A/B) = P(AnB)/P(B)

And p(AnB) = P(A) * P(B)
Then doesn't that just give P(A) because the P(B) cancel...
14. (Original post by hogree)
Can anyone clear this up -
Do we ever have to use Interpolation with Normal Distribution? Or are we always allowed to round to the nearest value? (Say, we're given 1.5833 and not 1.58 or 1.59).
how would we use interpolation in the normal distribution? I just use the z value that the probability is closest to

eg phi (0.68) = 0.7517 and phi (0.67) = 0.7486 and i am working out the upper quartile ie 0.75 i would use phi (0.67) as it is 0.0014 away vs 0.0017 ie it is closer
15. (Original post by tazza ma razza)
wtf - the exam is tomorrow how do you not know this....
Sorry... Do we need to know what a box and whisker is then or not?
16. (Original post by hogree)
You always use the Class Boundaries, but remember to always look at whether the Class Boundaries are "Rounded" or "Exact Values". So for example, if they use Values where Classes "share numbers like this:

Then the Class Width is just 5. BUT if the Classes look like this...

Then you can see that there is rounding going on. So actually the Class Width is not 41-45, but 40.5-45.5. Which is a Class Width of 5.
why is the first class width 40.5-45.5 instead of 41-45.5
and is the last one 90.5-150 or 90.5-150.5 ???
17. (Original post by haes)
why is the first class width 40.5-45.5 instead of 41-45.5
and is the last one 90.5-150 or 90.5-150.5 ???
Because in the question it says "To the nearest minute". Thus it applies to all of the values within the class boundaries. The last one is 90.5-150.5 .

(Original post by tazza ma razza)
how would we use interpolation in the normal distribution? I just use the z value that the probability is closest to

eg phi (0.68) = 0.7517 and phi (0.67) = 0.7486 and i am working out the upper quartile ie 0.75 i would use phi (0.67) as it is 0.0014 away vs 0.0017 ie it is closer
I guess the way to do it wold be instead of rounding as you say, do 0.67 + (14/31)*1 . But I do believe you're right. I think Interpolation is not necessary and we are able to round the nearest value.

(Original post by Siddhart1998)
This is gonna sound dumb but:If P(A/B) = P(AnB)/P(B)And p(AnB) = P(A) * P(B)Then doesn't that just give P(A) because the P(B) cancel...
That's right but that's ONLY TRUE when the events are independent. After all, if you think of, if A and B are independent. A|B (A given B) is basically asking for A since there's no link in independent events.
18. (Original post by Siddhart1998)
Sorry... Do we need to know what a box and whisker is then or not?
yh you need to know both and how to compare 2 plots
19. (Original post by tazza ma razza)
yh you need to know both and how to compare 2 plots
Isn't a Box and Whisker Plot the same as a Box Plot?
20. (Original post by tazza ma razza)
yh you need to know both and how to compare 2 plots
Thanks except there is nothing on "whiskers" in the textbook hence why I was asking aha

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