A new topic for me, but you explained it so well as always. I was so confused initially reading the example questions but had a few “aha moments” during the lecture. So thank you for helping me to get my head around it. 馃檪

sir,in determining the proportions of what is less than a mean(say 9.8 as in example2) can you work with the value directly by dividing with SD as done for those above and then change to standard form. like 9.8/0.2=49(4.9*10^1 then look up in the table just like in logarithms and indices).

Hi sir,
Please explain this questions.
Q1)A normal distribution has a mean of 75and a variance of 25.
what is the upper quartile of this distribution?
Q2)A normal distribution has mean of 150,and a standard deviation of 20.
80% of the distribution is below which of the following (approximately)?

In future please ask this kind of question in the Ask the Tutor forum, and not as a comment on a lecture.

For the first question you need to find the value that has a probability of 25% (0.25) of being above the mean.
For the second question you need to find the value that has a probability of 80 – 50 = 30% (0.30) below the mean.

In both cases you need to work backwards in the tables to find the number of standard deviations (the z score) in the way I explain in the lectures.

FinKi says

A new topic for me, but you explained it so well as always. I was so confused initially reading the example questions but had a few “aha moments” during the lecture. So thank you for helping me to get my head around it. 馃檪

John Moffat says

Thank you for your comment 馃檪

ABDULLAHI312 says

sir,in determining the proportions of what is less than a mean(say 9.8 as in example2) can you work with the value directly by dividing with SD as done for those above and then change to standard form. like 9.8/0.2=49(4.9*10^1 then look up in the table just like in logarithms and indices).

John Moffat says

No. You can only use the tables directly to find the proportion between the mean and some other value (here, between 10 cms and 9.8 cms).

faizalcs says

Hi sir,

Please explain this questions.

Q1)A normal distribution has a mean of 75and a variance of 25.

what is the upper quartile of this distribution?

Q2)A normal distribution has mean of 150,and a standard deviation of 20.

80% of the distribution is below which of the following (approximately)?

John Moffat says

In future please ask this kind of question in the Ask the Tutor forum, and not as a comment on a lecture.

For the first question you need to find the value that has a probability of 25% (0.25) of being above the mean.

For the second question you need to find the value that has a probability of 80 – 50 = 30% (0.30) below the mean.

In both cases you need to work backwards in the tables to find the number of standard deviations (the z score) in the way I explain in the lectures.

iza1 says

what if the z score for 10 and 10.4 is 2. 96

then the area from curve table that we would choose is 2.906 ‘s = 0 .4985 ?

iza1 says

Hi SIr,

Can you please explain again what you meant by area of bar is proportional to frequency.

I cant seem to understand it