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S1 Doubt

So, I have a calculator that can find out values from a standard distribution graph without the need to look at the tables. My question is that will I lose one mark, if a question comes up like this :

Find P(Z < -0.2352941176)

For this you need to do do 1 - P(Z > 0.2352941176)

Instead I can input P(-0.2352941176) into my calc directly and get the correct answer without needing to write the step in bold.

Will I lose a mark if I miss out the step in bold?
Reply 1
Avatar for Zacken
Zacken
22
Original post by Blazyy
Will I lose a mark if I miss out the step in bold?


No, you won't.
Reply 2
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Blazyy
OP
3
Original post by Zacken
No, you won't.


Thanks a bunch, I have a few more doubts, do you mind?
Reply 3
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Zacken
22
Original post by Blazyy
Thanks a bunch, I have a few more doubts, do you mind?


Go ahead.
Reply 4
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Blazyy
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3
Original post by Zacken
Go ahead.


How do you do part b of this question?
http://prntscr.com/ba68ia
Reply 5
Avatar for Zacken
Zacken
22
Original post by Blazyy
How do you do part b of this question?
http://prntscr.com/ba68ia


P(X < 1) = 0.05 ===> P(Z < (1 - mean)/standard deviation) = 0.05

And you know that P(Z < z) = 0.05 means that z = -1.6449 or w/e

So equate (1- mean)/s.d to -1.6449 and solve.
Reply 6
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Blazyy
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Original post by Zacken
And you know that P(Z < z) = 0.05 means that z = -1.6449 or w/e


I don't get this part
Reply 7
Avatar for MathsSir
MathsSir
1
Edexcel explicitly say on te front of their papers that you will not get full marks for a question unless the method is shown.
Reply 8
Avatar for Zacken
Zacken
22
Original post by Blazyy
I don't get this part


What don't you get? Your percentage points table shows that P(Z > 1.6449) = 0.05.

You have P(Z < z) = 0.05. Convert this to P(Z > -z) = 0.05. Then -z = 1.6449.
Reply 9
Avatar for Blazyy
Blazyy
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3
Original post by Zacken
What don't you get? Your percentage points table shows that P(Z > 1.6449) = 0.05.

You have P(Z < z) = 0.05. Convert this to P(Z > -z) = 0.05. Then -z = 1.6449.


Okay I understand, thanks.
The standard normal distributions give values for P(Z<z) where z>0. The probability distribution is symmetric about z=0 so we only need half the values.
If you want P(Z<z) = 0.05, then z must be negative. Sketch the distribution to see this. Then
P(Z<z) = 0.05 (where z is negative) is the same as P(Z>z) = 0.05 (z is now the positive value) which is the same as 1 - P(Z<z) where z is the positive value. My advice is to keep sketching the distribution for each question, shade in the bits for which you are given information and eventually you will be able to do the questions without a graph.
Reply 11
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Zacken
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Original post by Blazyy
Okay I understand, thanks.


No problem.

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