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# S2 ikkle questions watch

1. Just a few things I want to clear up...

In the following take « to be less than or equal to:

p(-0.92 « Z « 1.20)
≈ 0.7055-0.7070

On the 2nd line I am not sure where they got these numbers from - I look in tables for 1.20 and 1-(-0.92) and obviously they do not correspond. What am I doing wrong?

Earlier in the same question:

p(90 « X < 105) ≈ p(89.5 « Y « 104.5)

I know this to be continuity correction, but would like to know why in this instance you take away 0.5 for both, and why the < from the first bracket changes to a « but the « from the first bracket stays the same.

Small questions I know, but the little things tend to put me off big time

W.
2. i worked this out from the book and their examples and seems to be right...i hope

P(x less than or equal to 12) gives P(less than or equal to 12.5)
P(x < 12) gives P(less than or equal to 11.5)
P(x greater than or equal to 12) gives P(x great than or equal to 11.5)
P(x > 12) gives P(x great than or equal to 12.5)

P(x = 12) gives P(11.5 greater than or equal to X less than or equal to 12.5) depending on accuracy
3. For the continutity correction try looking at these http://www.math.mcmaster.ca/peter/s2...t_correct.html and http://www.mathsrevision.net/alevel/...ribution2.php. The general idea is that for discrete probabilities we need to approximate the 'area' (which corresponds to probability) of a rectangle using the normal curve, the diagrams given in the links should help explain this.

As for applying continuity correction for <= and <; first of all we have for example

P(X <= 12) goes to P(X < 12.5)

Then, for a discrete random variable we have

P(X < 13) = P(X <= 12) which goes to P(X < 12.5)
4. oki doki, I think I understand the continuity correction now

Any ideas on the 1st part of my question anyone???

Thanks again,

W.
5. (Original post by wonderboy)
Earlier in the same question:

p(90 « X < 105) ≈ p(89.5 « Y « 104.5)
this is saying less than 105 and greater/equal to 90

the way I do it is if theres an 'extra' bit to the sign, then it 'corrects' the wrong way. so, greater equal will correct down (instead of correcting up) whereas less than will correct down.
6. Thanks kik, I just need to get it straight in my head

p(-0.92 « Z « 1.20)
≈ 0.7055-0.7070

Anyone know where they got 0.7055 and 0.7070 from ??
7. (Original post by wonderboy)
Thanks kik, I just need to get it straight in my head

p(-0.92 « Z « 1.20)
≈ 0.7055-0.7070

Anyone know where they got 0.7055 and 0.7070 from ??
No, I dont see where they got those values from.

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