Standard Deviation & Probability

[paddyk]

Hi,

I can calculate the probability of the mean occurring using the NORM.DIST function, ie 50% probability of being the mean, 28% probability of being the mean +1,etc.

What I need to get though is what mean value (mean +1, mean +1.1 etc) is for a given probability %. For example at 25%/50%/75%/100% probability the mean value is x.

Can anyone help?

Thanks in advance.

 

[shg]

	A​	B​	C​
1​	x​	​	​
2​	25%​	-0.67449​	B2: =NORM.S.INV(A2)
3​	50%​	0​
4​	75%​	0.67449​

 

[paddyk]

Thanks, is there any way I can apply that to a data set? So that using the mean and standard deviation I can say the mean at 25% probability?

My data set includes a number of days per record, I have calculated the probability of the day's being mean+1,mean-1,etc but what would be more useful for me would be to reverse that calculate the number of days for the data set at a set probability % (25,50,75, etc). So instead of getting a 41% probability of being mean-1, it would show at 25% probability the mean would be 4.2.

Hope that makes sense!

 

[shg]

I don't follow that. Please provide a clear example.

 

[paddyk]

What I have calculated is in rows 1-5, where for example, for code 3660 there's a 22% probability the number of days will be equal to or less than mean-1 (2.55 days) and a 78% probability the number of days will be equal to or less than mean+1 (4.55 days).

What would be more useful is if I could calculate the number of days at banded probabilities, rows 7-9. For example, for code 3660 at 25% probability the number of days will be x, at 75% probability the number of days will be y.

	A	B	C	D	E	F	G	H	I
1	Code	Mean	Std Deviation	Probability Mean-2	Probability Mean-1	Probability Mean	Probability Mean+1	Probability Mean+2
2	3660	3.55	1.3	6%	22%	50%	78%	94%
3	455	8.8	11.36	43%	46%	50%	54%	57%
4	3910	3.83	1.43	8%	24%	50%	76%	92%
5	3661	9.85	10.77	43%	46%	50%	54%	57%
6
7	Code	Mean	Std Deviation	Mean @ 25% Probability	Mean @ 50% Probability	Mean @ 75% Probability
8	3660	3.55	1.3	?	3.55	?
9	455	8.8	11.36	?	8.8	?

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[shg]

	A​	B​	C​	D​	E​	F​	G​
7​	Code​	Mean​	Std Deviation​	25%​	50%​	75%​
8​	3660​	3.55​	1.30​	2.67​	3.55​	4.43​	D8: =NORM.INV(D$7, $B8, $C8)
9​	455​	8.80​	11.36​	1.14​	8.80​	16.46​

 

[paddyk]

Thanks a million! Worked perfectly.

 

[shg]

You're welcome.