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# Edexcel S2 Exam Paper January 2011 14/01/11 Watch

1. (Original post by jit987)
The variable T Distributed by N(8, 4), why does P(T = 5) = 0?
Isn't it meant to be P(4.5 < T < 5.5)?
Because with a normal distribution, the probability that it occurs at a single point is always zero, you can only use a normal distribution when investigating the probability a value is above, below, between two certain points, i.e. an inequality. When you change the probability, like from P(T=5) to P(4.5 < T < 5.5), you're thinking about when you change the type of distribution, or something like that, it's been a while.
2. (Original post by jit987)
The variable T Distributed by N(8, 4), why does P(T = 5) = 0?
Isn't it meant to be P(4.5 < T < 5.5)?
This only applies if you are approximating a discrete distribution like binomial or poisson onto a continuous distribution like the normal distribution. In this question you aren't approximating anything. The question STATES that T follows a normal distribution. As the normal distribution is continuous the probability that it takes any particular value is zero. Think about it: the probability is represented by the area under the graph for the pdf of the normal distribution. At one particular value this is the area of a line which is zero.
3. (Original post by Noble.)
Because with a normal distribution, the probability that it occurs at a single point is always zero, you can only use a normal distribution when investigating the probability a value is above, below, between two certain points, i.e. an inequality. When you change the probability, like from P(T=5) to P(4.5 < T < 5.5), you're thinking about when you change the type of distribution, or something like that, it's been a while.
it clearly has
4. (Original post by anshul95)
it clearly has
What's wrong in that statement?
5. (Original post by Noble.)
What's wrong in that statement?
I was being sarcastic - now that I read back I wasn't being clear enough sorry. You actually explained it very well for someone who potentially hasn't done this for a year.
6. (Original post by anshul95)
I was being sarcastic - now that I read back I wasn't being clear enough sorry. You actually explained it very well for someone who potentially hasn't done this for a year.
Haha, I was interested to see how much of it was nonsense after not doing it for a while. I have a pretty good memory, so tend not to forget maths, even if it is useless.
7. (Original post by Noble.)
Haha, I was interested to see how much of it was nonsense after not doing it for a while. I have a pretty good memory, so tend not to forget maths, even if it is useless.
Thats what people say about me as well. Its probably because we enjoy maths so much its not like doing work - well thats how I feel when I'm doing something relating to maths.
8. (Original post by Noble.)
...

(Original post by anshul95)
...
Thanks! Well explained
9. how do you know when too use the continuity correction? and if so when do you -0.5 and +0.5??
thanks
10. (Original post by gupta21)
how do you know when too use the continuity correction? and if so when do you -0.5 and +0.5??
thanks
you'd use the continuity correction when you're approximating a poisson or binomial distribution with a normal distribution.

Example:
when the discrete distribution is
P(X<17)
continuous would be
P(X<16.5)

BUT

when the discrete distribution is
P(X>17)
continuous would be
P(X>17.5)

ALSO: because for a normal distribution you cannot approximate 'equals to' so,
when the discrete distribution is
P(X?17) [The '?' should be 'less than/equal to' everytime I save it changes to '?' sorry about that )
continuous would be
P(X<16.5)

So when you are calculating the probability for X 'LESS THAN', you'd use the lower bound of X
and
when you are calculating the probability for X 'GREATER THAN', you'd use the upper bound of X

Hope that helps
Good luck!
11. thank you so much 'twiddle-doo'
i finally get it
12. (Original post by gupta21)
thank you so much 'twiddle-doo'
i finally get it
no probs
13. Hope Sampling Distribution question pops up
14. (Original post by jit987)
Hope Sampling Distribution question pops up
I just hope no 'define blah blah' comes up
15. (Original post by jit987)
Hope Sampling Distribution question pops up
Same! Although I still cannot remember how to define it in a statement, aghhh

(Original post by Noble.)
....
This makes sense, thanks!!!!
16. For the continuity correction

when the discrete is P(x>17)
isnt the continuous P(x>16.5) the lower band?
17. (Original post by Foxer27)
For the continuity correction

when the discrete is P(x>17)
isnt the continuous P(x>16.5) the lower band?
yeah i had the same question when i was doing this normal distribution but my teacher said no for P(X>17) would be P(X>17.5)
and the reason being for this is because we want values above 17 and 16.5 can be rounded DOWN to 16 than 17

hope that helps
18. (Original post by Foxer27)
For the continuity correction

when the discrete is P(x>17)
isnt the continuous P(x>16.5) the lower band?
It is not

If it would have been , then it would be

If I'm wrong please correct me
19. (Original post by jit987)
If you can handle those from 01-10 and solomon's very well, then there shouldn't be any problem. How did you find the solomon's?
solomons were ok...i hope nothing like P(4<X<5) comes because i alwys mess up those....any particular tricks to learn how to solve those??
20. (Original post by Geniusmiss)
solomons were ok...i hope nothing like P(4&lt;X&lt;5) comes because i alwys mess up those....any particular tricks to learn how to solve those??
Is that for Uniform distribution questions, like in some interval (0,20) then work out P(4<x<5). Or are you talking about poisson/binomial/cdf/pdf?
For a PDF question if the questions says P(x=5) is it always 0 or is that only for CDF?

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