Is it possible to find the SD without the data?

[stonecoldgen]

I have the mean and I have the value at a percentile (99th percentile to be specific). I am just wondering if it's possible to find the SD. Actually, what I am trying to find is the value at the 90th percentile (I think I need the SD to do it, but maybe just maybe there's another method...)


Just in case, the mean is 3.8 and the value at the 99th percentile is 4.64

 

[mathman]

You need to know the shape of the distribution. If it is normal you can get the S.D. (σ)from the data given by looking up the value for a when aσ = .99. I get a ≈ 2.33, but I suggest you check it. σ = (4.64 - 3.8)/2.33.

 

[stonecoldgen]

Oh yeah, I forgot to say it's a normal distribution. I used the invNorm function to see which standard deviation best models the situation, so I found out that 0.36 does it pretty well. So the value I got for the 90th percentile is more of an estimate (4.26), but I think it fits well enough.