# S2 ikkle questions

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#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.
0
15 years ago
#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
0
15 years ago
#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)
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#4
oki doki, I think I understand the continuity correction now Any ideas on the 1st part of my question anyone???

Thanks again,

W.
0
15 years ago
#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.
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#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 ??
0
15 years ago
#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.
0
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