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The Edexcel S1 (18/05/12 - AM) Revision Thread

Reply 780

Akbar2k7

20

Original post by Arnewaise

When you have like P(x=1.5) or F(1.5), what should you do? Do they want you to round the number up or down?
cheers

You Always round down.

Reply 781

Cuzzz

Original post by I love Vagina

45/2 = 22.5. Hence, round up to 23 and find the the 23rd value which is 53. It was not an integer so he rounded up :smile:

is it not n+1 /2?

Reply 782

jerichi

HELP

A spinner is designed so that the score S is given by the following probability distribution.
s 0 1 2 4 5
P(S = s) p 0.25 0.25 0.20 0.20
(a) Find the value of p.
(2)
(b) Find E(S).
(2)
(c) Show that E(S2 ) = 9.45
(2)
(d) Find Var(S).
(2)

Tom and Jess play a game with this spinner. The spinner is spun repeatedly and S counters
are awarded on the outcome of each spin. If S is even then Tom receives the counters and
if S is odd then Jess receives them. The first player to collect 10 or more counters is the
winner.

(e) Find the probability that Jess wins after 2 spins. (2)
(f) Find the probability that Tom wins after exactly 3 spins. (4)
(g) Find the probability that Jess wins after exactly 3 spins. (3)

I'm not sure what to do for (e) (f) and (g)...please help me! :/

Reply 783

SHAH :)

Good luck everyone!! :smile:

Reply 784

otrivine

14

Original post by SHAH :)

Good luck everyone!! :smile:

http://www.edexcel.com/migrationdocuments/QP%20GCE%20Curriculum%202000/January%202008/6683_01_que_20080115.pdf
how do you do question 2)a) 1/2(n+1) ??

Reply 785

I love Vagina

Original post by Cuzzz

is it not n+1 /2?

I thought that was when finding the mean in discrete uniform distributions? But also, the question wants the median, so you would divide by 2. Similarly, you would divide by 4 to find Quartile 1. Anyway, good luck! :biggrin:

Reply 786

Killer_94

Original post by otrivine

http://www.edexcel.com/migrationdocuments/QP%20GCE%20Curriculum%202000/January%202008/6683_01_que_20080115.pdf
how do you do question 2)a) 1/2(n+1) ??

to work out the mean add up all the values of x and divide them by 12
and then to work out S.D you can just use the formula....

Reply 787

Spellstheend

Original post by arnab

No, not really :mad:

Ok, i'll go through it step by step and try and break it down so its clearer

So the question says find the probability that the employer lives close to the company, given that they are a manager

Now, the probability is for this person being a manager AND living close.
The fact they say 'given that they are a manager' means that the other options - administration and production are completely irrelevant and therefore ignored. The groups of people you are looking at is narrowed to only the managers as soon as the it says 'they are a manager'. The probability we are finding is to do with managers only, because they have said they are picking from the group of managers who lives close.

Now we have narrowed it down to the first row only as all the people in the first row, the 6 who live close and the 14 far away are all managers.

Now we just want the people who live close, which is 6. This 6 is out of the group of total managers, which is 4 + 16

So in conclusion, 6 out of 20 managers live close, so that's 0.3

The point I think you are missing is it is conditional probability, as soon as it says 'given...' you have to immediately discount anything that isn't part of the given, in this case being an administrator or producer is not being a manager.

If you don't fully understand still, which points are you not following?

Reply 788

otrivine

14

Original post by Killer_94

to work out the mean add up all the values of x and divide them by 12
and then to work out S.D you can just use the formula....

no not the mean it is part a ) the median and quartiles cant get it ? makes no sense

Reply 789

Spellstheend

Original post by jerichi

HELP

A spinner is designed so that the score S is given by the following probability distribution.
s 0 1 2 4 5
P(S = s) p 0.25 0.25 0.20 0.20
(a) Find the value of p.
(2)
(b) Find E(S).
(2)
(c) Show that E(S2 ) = 9.45
(2)
(d) Find Var(S).
(2)

Tom and Jess play a game with this spinner. The spinner is spun repeatedly and S counters
are awarded on the outcome of each spin. If S is even then Tom receives the counters and
if S is odd then Jess receives them. The first player to collect 10 or more counters is the
winner.

(e) Find the probability that Jess wins after 2 spins. (2)
(f) Find the probability that Tom wins after exactly 3 spins. (4)
(g) Find the probability that Jess wins after exactly 3 spins. (3)

I'm not sure what to do for (e) (f) and (g)...please help me! :/

I briefly explained it here:
http://www.thestudentroom.co.uk/showthread.php?t=1949172&page=25&p=37595881#post37595881

If you have any more questions about it then just ask

Reply 790

robthefool

12

Original post by Cuzzz

is it not n+1 /2?

No no no no no.

First divide by 2. if the number is an integer, eg 23, then round up to 23.5. Likewise if it is 17 you would round up to 17.5. So the median would be the 23.5th position or the 17.5th position.

When you divide by 2 and it isn't an integer, always round up. Eg 17.2 would be 18 and 16.8 would be 17

Reply 791

Killer_94

Original post by otrivine

no not the mean it is part a ) the median and quartiles cant get it ? makes no sense

oohhh its actually pretty simple... put all the values of x in the right order (smallest to biggest) and then to work out the median you divide 12 by 2 which is the 6th value, so you pick the 6th and the 7th value and divide by two.
then to work out the L.Q you divide 12 by 4... and you pick the 3rd and 4th value and divide by two. similar for U.Q

Reply 792

TaraBelle

10

is symmetrical skew and no skew the same thing? :s-smilie:

Reply 793

SHAH :)

Original post by otrivine

http://www.edexcel.com/migrationdocuments/QP%20GCE%20Curriculum%202000/January%202008/6683_01_que_20080115.pdf
how do you do question 2)a) 1/2(n+1) ??

for the mean you add all the x values and divide by 12.
for the standard deviation they give the x squared number so you divide that by 12 and minus the mean squared and then you square root it

im not sure what you mean by 1/2(n+1) but what i said gives you the correct answer :smile:

Reply 794

rommy123

Just going through Probability and Normal distribution, Which I find really hard :/
Hope there is only one question of each tomorrow,

Good luck everyone :smile:

Reply 795

jerichi

Original post by otrivine

http://www.edexcel.com/migrationdocuments/QP%20GCE%20Curriculum%202000/January%202008/6683_01_que_20080115.pdf
HI QUESTION 2)A) I DONT GET HOW THEY DID IT CAN YOU NOT USE THE FORMULA 1/2(N+1)? OR 1/4(N+1)

Well this case is different. It's pretty straightforward to find in this one cause you have the total number which is 12 and the data listed. Rearrange the data.
The for median use n/2 to find the position of the value. Since the position is an even number you take the value in that position (once the values are arranged in ascending order) and the value after. Add then and then divide. You do the same for upper quartile (3n/4) and lower quartile (n/4) and follow the same rule as that of median. Does that make sense?

For eg the question itself.
Once the values are rearranged they list like this;
125, 160, 169, 171, 175, 186, 210, 243, 250, 258, 390, 420

Median: n/2=6
6th value is 186 but since it's even you take the value after as well.
Hence: Median= (186+210)/2
= 198

Does this help?

Reply 796

SHAH :)

does anyone have the jan 2012 paper??

Reply 797

Frodo Baggins

STATISTICAL MODELS

1. A Real world observation is made
2. A suitable mathematical model is devise
3. Predictions are made
4. Experimental data is collected
5. Comparisons of observational outcomes
6. Statistical tests are then used to see ho well the model represents the real world
7. The mathematical model is refined.

Reply 798

Killer_94

Original post by SHAH :)

does anyone have the jan 2012 paper??

you can find all the papers here:
http://mathspapers.co.uk/edexcel.html

Reply 799

otrivine

14

Original post by jerichi

Well this case is different. It's pretty straightforward to find in this one cause you have the total number which is 12 and the data listed. Rearrange the data.
The for median use n/2 to find the position of the value. Since the position is an even number you take the value in that position (once the values are arranged in ascending order) and the value after. Add then and then divide. You do the same for upper quartile (3n/4) and lower quartile (n/4) and follow the same rule as that of median. Does that make sense?

For eg the question itself.
Once the values are rearranged they list like this;
125, 160, 169, 171, 175, 186, 210, 243, 250, 258, 390, 420

Median: n/2=6
6th value is 186 but since it's even you take the value after as well.
Hence: Median= (186+210)/2
= 198

Does this help?

i got it

so if it comes to 6th value do we just count and get the 6th value but if it was 6.75 something we add the two and divide by 2 ?correct

The Edexcel S1 (18/05/12 - AM) Revision Thread

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