Edexcel S1 A Level Maths

Just to check for the midpoints

0-9
10-19
20-29

For 0-9 would it be 0-9.5?
So 9.5 / 2 to get the midpoint??

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Reply 442

Livaren

Could someone try quickly explain to me how to work out percentiles using interpolation?

I know how to use interpolation to work out the median/quartiles etc.

Reply 443

help-me333

12

Original post by dancergirl

I have a question about upper and lower quartiles.
If I had 30 pieces of data, I would do 30+1/2 to give me 15.5 and would then find the 15.5th value which would be the median (right?)
Then to work out Q1 I would do 15.5/2 which gives me 7.75, so would I find the 7.75th value or would I round it to something else?
I've just been guessing this whole time haha

I'm curious of this too

Reply 444

User995789

14

Original post by Livaren

Could someone try quickly explain to me how to work out percentiles using interpolation?

I know how to use interpolation to work out the median/quartiles etc.

Same.

Reply 445

Anyone know where I can get the markscheme for s1 from jan(this year) just doing it now can't get a working link anywhere

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Reply 446

help-me333

12

Original post by Livaren

Could someone try quickly explain to me how to work out percentiles using interpolation?

I know how to use interpolation to work out the median/quartiles etc.

think of the median as the 50th percentile

n x 0.5

so to work out say the 60th percentile

you'll do n x 0.6

Reply 447

Didierr

This is from Jan 2011 question 6 and I have no clue how question e and f, if anyone has any clue please help!

http://www.edexcel.com/migrationdocuments/QP%20GCE%20Curriculum%202000/January%202011%20-%20QP/6683_01_que_20110114.pdf
6. The discrete random variable X has the probability distribution
x 1 2 3 4
P(X = x) k 2k 3k 4k
(a) Show that k = 0.1
(1)
Find
(b) E(X)
(2)
(c) E(X 2)
(2)
(d) Var(2 – 5 X )
(3)
Two independent observations X1 and X2 are made of X.
(e) Show that P(X1 + X2 = 4) = 0.1
(2)
(f) Complete the probability distribution table for X1 + X2
(2)
y 2 3 4 5 6 7 8
P(X1 + X2 = y) 0.01 0.04 0.10 0.25 0.24

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___________________________________________________________________________

Reply 448

Original post by Blueray2

Same.

Multiply the total number (n) by x/100 x, being the percentile (e.g. Percentile 33 would be 33/100) then use interpolation from there

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Reply 449

ManWithTwoHorns

I hate Normal Distribution so much, seriously, nothing is easy about that. It's just a major headache. Don't even know what to look for half of the time.

Reply 450

Original post by Didierr

This is from Jan 2011 question 6 and I have no clue how question e and f, if anyone has any clue please help!

http://www.edexcel.com/migrationdocuments/QP%20GCE%20Curriculum%202000/January%202011%20-%20QP/6683_01_que_20110114.pdf
6. The discrete random variable X has the probability distribution
x 1 2 3 4
P(X = x) k 2k 3k 4k
(a) Show that k = 0.1
(1)
Find
(b) E(X)
(2)
(c) E(X 2)
(2)
(d) Var(2 – 5 X )
(3)
Two independent observations X1 and X2 are made of X.
(e) Show that P(X1 + X2 = 4) = 0.1
(2)
(f) Complete the probability distribution table for X1 + X2
(2)
y 2 3 4 5 6 7 8
P(X1 + X2 = y) 0.01 0.04 0.10 0.25 0.24

___________________________________________________________________________
___________________________________________________________________________

For e all it wants you to do is find all the ways you could make 4, basically the event happens twice and what is the probability it makes 4, so get all the way you could make 4 (2+2,1+3 etc) and multiply them up and add them together, you should get 0.1. After that you should be able to figure out f( same idea)

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Reply 451

User995789

14

Original post by BarryIsMyName

Multiply the total number (n) by x/100 x, being the percentile (e.g. Percentile 33 would be 33/100) then use interpolation from there

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Can you show that paper please :biggrin:

Or tell me a question of it :smile:

Reply 452

Livaren

Original post by Didierr

This is from Jan 2011 question 6 and I have no clue how question e and f, if anyone has any clue please help!

http://www.edexcel.com/migrationdocuments/QP%20GCE%20Curriculum%202000/January%202011%20-%20QP/6683_01_que_20110114.pdf
6. The discrete random variable X has the probability distribution
x 1 2 3 4
P(X = x) k 2k 3k 4k
(a) Show that k = 0.1
(1)
Find
(b) E(X)
(2)
(c) E(X 2)
(2)
(d) Var(2 – 5 X )
(3)
Two independent observations X1 and X2 are made of X.
(e) Show that P(X1 + X2 = 4) = 0.1
(2)
(f) Complete the probability distribution table for X1 + X2
(2)
y 2 3 4 5 6 7 8
P(X1 + X2 = y) 0.01 0.04 0.10 0.25 0.24

___________________________________________________________________________
___________________________________________________________________________

All you do for e, is note all the combinations of two x values that you can add to get 4, times them together and add all combinations should equal 0.1.

For f start with 8 since it has small combinations, the only combo from 2 numbers is 4 and 4, times them and thats your porbability for 8 and do 1- all the probabilities to find the 5th one.

Reply 453

shyamshah

12

You know when you are doing linear interpolation and you have found the cumulative frequency and know the range of values where the median is, what value do you use as the median for the interpolation, is there a way of working it out?

Reply 454

Original post by help-me333

I'm curious of this too

Btw I wasn't taught to use the n+1 method
Learnt without needing it if you stick to these rules

Discrete/ungrouped data
So you did the n/2
Lets say = x
If x is a whole number, find that term and the term above

If x is not a whole number always round up even if less than .5

Grouped data then straight to interpolation

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Reply 455

Didierr

Original post by dancergirl

I have a question about upper and lower quartiles.
If I had 30 pieces of data, I would do 30+1/2 to give me 15.5 and would then find the 15.5th value which would be the median (right?)
Then to work out Q1 I would do 15.5/2 which gives me 7.75, so would I find the 7.75th value or would I round it to something else?
I've just been guessing this whole time haha

if you had 30 pieces of data you would find the median by doing 30/2 which is 15. then add 0.5 if a whole number so find the 15.5th value, if a decimal e.g. 7.2, 7.5, 7.8 etc your round right up to the next number for whatever the decimal and find the 8th value. Do the same for Q1 and Q3... if that makes sense

Reply 456

Original post by BarryIsMyName

Anyone know where I can get the markscheme for s1 from jan(this year) just doing it now can't get a working link anywhere

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Search for jan 13 s1 model answer arsey on tsr
Or something like that ...

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Reply 457

Original post by dancergirl

I have a question about upper and lower quartiles.
If I had 30 pieces of data, I would do 30+1/2 to give me 15.5 and would then find the 15.5th value which would be the median (right?)
Then to work out Q1 I would do 15.5/2 which gives me 7.75, so would I find the 7.75th value or would I round it to something else?
I've just been guessing this whole time haha

Answered your question in another post :smile:

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Reply 458

username1129244

15

Can someone confirm these for me:

Explanatory variable is basically the independent variable and we give a reason saying we can control it.

Normal distribution - P(X<w)=0.2 becomes P(X>w)=-0.2

Coding affects the mean as usual, S.D. through multiplying/dividing, Variance + Sxx or Syy by (multiplying number)^2

Reply 459