The presentation on Normal Distribution is here. Have a look please D.
Ex 9A and 9B for Weds please, and the S1 for Friday. Deola – please pick one up from my desk on Monday.
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Mr Brewin’s Year 12 Stats31 comments to Mr Brewin’s Year 12 Stats |
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Im really stuck on question 9 of excericise 9C, what do i have to do with it to find what a and b are?
I don’t have the book Becky! Can you give me a few more details?
haha ok… it says: the random variable Y~N(100,15 squared)
find the value of a and the value of b such that
a) P(Y>a) = o.975
b) P(Y<b) = 0.10
Hi Becky,
The numbers in the N bracket give you the mean and variance (SD squared).
You will need to standardise this then use the tables.
Does that help to start you off?
umm not really :s… do i have to use the equation z = x – mean/standard dev and go down that route.. cause i did that and i somehow ended up with a number that was much larger than that in the answer book
Yes. Remember that to do greater than, you need to do 1-less than
yeeah so i ended up with 1 – 0.975 = 0.025
and then i couldnt look it up in the table because it would be a negative
Use the symmetry of the curve about zero to get round negative values.
Sketch the curve and draw the region you want. Work it out at the other end using 0.975.
i feel like the dumbest person ever… i am just having a mind block right now. So do i look up 0.975 in the table?
Have you any idea how long it is since I did any stats lol? I am struggling here too. What’s the answer for the a value so I can check my working?
PS Hope it’s 70.6
yeah it isss… omgggggggggggg how did you do it cause i am ending up with a ridiculous number compared to that! oh i wish i could do this! you have no idea how stressful this is becoming lol
YAY!!!!!!!!!
I’ll try to explain what I did. You were right with the 0.025 bit, but it’s down the left hand end of the graph. The bit at the other end is the same size and has an area left of the line of 0.975. Find 0.975 in the body of the table – it’s at 1.96 which means, at the other end, the value is -1.96. Therefore (a – 100)/15 = -1.96
Solve that and you get a = 70.6
ahhhh i was soooo close!!! I didnt make it a negative value and so i was just getting a really high figure.. oooh it was such a struggle. Thankyou SO much!!
You’re welcome – I’ve even impressed myself hehehe
Mrs T have you seen my comments on the sixth form bit??
No! If you post on a password protected bit, they don’t show in the list – you need to use a different thread.
oo no i thought you were ignoring me for a sec. ok i’ll re-post it
Put it on year 13 decisions
hi sorry i dont know where to write this but i dont know what the password for the protected S1 papers is :s please could someone let me know? thanks
Jacynth
Err… can’t really say exactly, but what’s the local area our school is in? Try that!
Mr Brewin,
Sorry to bother you but in the stats textbook chp 5 mixed exercise for question 5b. If you did 6 x ×7 /15×3 /14 ×5 /13
or went through the whole tree diagram doing each route then adding together would you get the same answer?
Which way would you suggest?
Thanks
tried the thing you told Jacynth and couldn’t get in. Has the password been changed?
thanks
Hi Celia,
First letter is a capital.
JT
sorry to bother again, for the formula P(A)+P(B)- P(AnB)= P(AuB)
The u one and and n can be switched round?
thanks
Yes – cos it’s just change side, change sign for the two terms.
With lines of regression calculations
for the final answer a +bx should a and b be done to 3sf or 2dp?
Thanks
Hi Celia,
I’ve looked at a mark scheme and it said 3sf or better, so I would go with 3sf in the absence of any guidance.
JT
My final question or least I hope.
When dealing with median should you always add one and divide by 2. And if it is a even number adding one and dividing by 2 you get a .5 number so you add the number either side.
when dealing with the continuous data and doing interpolating and you have an even number should you use the .5 number or use the number you would get if you simpley divided by 2.
Finally for the upper and lower quartiles add one and divide by four (then times 3 for UQ). you get. 25. do you round up or down or do you add the numbers found to either side of the .25 area and divide by 2. same for continuous should you add one ?
Thank you Mrs Tibble been a big help this weekend sorting out the easy but for some reason bothering me bits .
Celia
Hi Celia,
Yes, add 1 before dividing by 2 or 4.
If you get .5, go halfway between the numbers each side of it.
If you get .25, go 1/4 of the way up the difference.
If you get .75, go 3/4 of the way up the gap.
Hope that makes sense!
Mr W is doing some S1 revision with the year 13 retakes tomorrow, so if you’re around feel free to pop in.
JT