Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Is it possible to find the SD without the data?

  1. Mar 18, 2013 #1
    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
  2. jcsd
  3. Mar 18, 2013 #2


    User Avatar
    Science Advisor

    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.
  4. Mar 18, 2013 #3
    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook