What % get 41-44 & 46-49 ?

[crappy]

LOL I cant remember anything from methods, did it nearly 2 years ago.

[stonecold]

Isnt PDF if you want to find what proportion get a fixed value, whilst CDF  will give you what proportion will get a range of values, hence the upper and lower bounds?

[m@tty]

First I did normPDF(50,30,7) and got 0.000962 which is 0.1%

Then I did what m@tty suggested and went:
normCDF(49.5,50,30,7) and got 0.000533 which is 0.05%  

This is confusing LOL.  And TI-Nspire rapes them all!  

For upper bound put something like or or another big number, the difference is insignificant as long as its from the mean.

[m@tty]

Isnt PDF if you want to find what proportion get a fixed value, whilst CDF  will give you what proportion will get a range of values, hence the upper and lower bounds?

When using a form of continuous probability like the bell curve probabilities are found by determining the area under the curve. The area under a curve at a single point is zero. Therefore you must always find the probability of a range of values occuring.

[stonecold]

That is better

normCDF(49.5,10^10,30,7) gives 0.00267 which is 0.27%

Thanks m@tty!

[kyzoo]

whats with the 49.5?

[m@tty]

It rounds up to 50.

[TrueTears]

Isnt PDF if you want to find what proportion get a fixed value, whilst CDF  will give you what proportion will get a range of values, hence the upper and lower bounds?

When using a form of continuous probability like the bell curve probabilities are found by determining the area under the curve. The area under a curve at a single point is zero. Therefore you must always find the probability of a range of values occuring.

PDF gives the "height" of the probability density function.

[vexx]

^ matty, so its more accurate if you put the up value as 1000 or something?
i just did for 44, and i got 2.2% and for 45 its 1.6%
would this be correct?

[m@tty]

I think so.
I remember reading in some VTAC publication [which I can't access now cause their site is screwed up] that Study scores are based upon a normal distribution of mean 30 and standard deviation 7 and is truncated at 0 and 50. I don't know exactly what that means, so .
EDIT:
Have you seen this?

For studies with large enrolments (1,000 or more):

• 2% of students will get a score on or above 45
• 9% of students will get a score on or above 40
• 26% of students will get a score on or above 35
• 53% of students will get a score on or above 30
• 78% of students will get a score on or above 25
• 93% of students will get a score on or above 20.

and 400th post!!

[vexx]

^ hmmm okay thanks.

isn't it odd then that it says 9% get 40+, when on the calc it says to 7.44%?

[m@tty]

I get 8.74%.
Which will round to 9%, which is what they show.

normcdf(39.5,1000,30,7)=0.08736

[vexx]

I get 8.74%.
Which will round to 9%, which is what they show.

normcdf(39.5,1000,30,7)=0.08736

oh woops i didnt do the 1000 thing.

thanks heaps!

[vexx]

... wasting time .__.

ah, i just think i did it wrong, because apparently you need to go .5 less? so for 45, its 44.5, which is 1.9%
gah...

edit, i made a real one, check out nova's thread...
http://vcenotes.com/forum/index.php/topic,21207.msg215008.html#msg215008

[kenhung123]

Competition is obvious at those numbers =.=