1
$\begingroup$

help.

Problem: Given a bounded Gaussian Distribution -- looking reproduce similar results i.e. same mean and standard deviation randomly.

Definition: Data set exhibits properties of a Gaussian distribution. Data set is bounded. Each value of the data set is an integer.

Let mu and sigma represent the mean and standard deviation. Let a and b represent the lower and upper bounds.

Comments: Tried to employ both rtnorm or rttruncnorm functions in R programming.

When I get a value from either of these functions, I'm simply rounding the result to the nearest integer. When I elect to unbound either function ie. from -Inf to Inf, I find that I can reproduce an experimental data set that is quite similar to the original data except for having data values that are obviously falling outside of the original boundaries [a,b], but yes do achieve the goal mu and sigma.

When I bound my result using these functions, the effective standard deviation is smaller as compared to the original standard deviation; I'm cutting away the head and tail of the distribution leaving an effective sigma that spans a smaller range.

While I appreciate that I can multiply and shift between truncated and non-truncated distributions using (X-mu)/sigma where X is a random value, the real goal is to reproduce an experimental truncated Gaussian distribution corresponding to the original truncated Gaussian data set.

For example, I tried simply increasing the goal standard deviation in rtnorm function for a bounded data set [a,b] to generate an effective standard deviation that is equivalent to the original standard deviation; however, this approach failed. This resulted in the standard deviation increasing somewhat but the effective standard deviation hit a ceiling falling short of the goal sigma thus never realizing the original standard deviation, mean and boundaries.

Remember, I am going on the premise that since I have an actual, real-world bounded Gaussian distribution with a given mean and standard deviation, I'm thinking that I can absolutely reproduce a similar randomly generated result.

Am I mistaken? What might I do differently?

I appreciate any insight that any of you might lend. Your help would really be welcomed and valued.

$\endgroup$
  • $\begingroup$ In the help page for rtnorm it says mean and sd are for the untruncated distribution. $\endgroup$ – Peter Flom - Reinstate Monica Apr 24 '13 at 23:57
  • $\begingroup$ What's your evidence that it is Gaussian, other than being rounded and bounded? Unless you've made a coding error, it sounds like you've shown that it isn't. Maybe give us some actual values of a,b, mu and sigma of the original data and a qqplot. $\endgroup$ – Peter Ellis Apr 25 '13 at 1:25
  • $\begingroup$ Thank you for the reply Peter. Actually, I think that you are right. After scratching my head further, I realized that all my observations didn't adhere to a Gaussian distribution strictly. 68% 1 standard deviation 95% 2 standard deviations 97.7% 3 standard deviations What I found was that the distribution was more like the following: 55% 1 standard deviation 100% 2 standard deviations Know, I guess, I need to figure what kind of distribution that I really have. Any suggestions? $\endgroup$ – Gideon Apr 25 '13 at 1:59
  • $\begingroup$ plot(density(x)); and library(MASS); truehist(x) are good starts. Unless you've made a coding error there must be something dramatic enough that it will show up visually, as it seems the variance of the population is more than of a uniform distribution and this should be visible. $\endgroup$ – Peter Ellis Apr 25 '13 at 2:22
2
$\begingroup$

As the variance of your simulated untruncated normal distribution increases the density gets flatter across the [a,b] interval. At its limit it becomes almost a uniform distribution hence with a maximum standard deviation of $\sqrt{\frac{1}{12}(b-a)^2}$. If the standard deviation of the population you are looking at is more than that, it must mean you have a bimodal distribution of some sort.

Even in that case (which means you don't have a truncated normal distribution) the maximum theoretical variance happens when $\mu$ is bang in the middle of $a$ and $b$ and the data is all at the extremes - half at the lower bound and half at the upper. The maximum possible standard deviation from a bounded distribution of any sort is $\sqrt{.5(a-\mu)^2+.5(b-\mu)^2}$ which reduces to $b-\mu$.

I would suggest you look at histograms and density line plots of your original data and question your starting assumption that comes from a truncated rounded Gaussian distribution. I would also compare its actual standard deviation with the two theoretical limits above. If it is less than the standard deviation of a uniform distribution you should be able to simulate a truncated normal distribution with that standard deviation, and you probably have a coding error. If it is more than that amount, you need to find an alternative model.

$\endgroup$
  • $\begingroup$ Thank you Peter for the second reply. Again, you are correct that the data doesn't entirely adhere to a Gaussian Distribution. I'm now going through the checklist using the reference for example -- "How to determine the probability distribution Type for your data?" ehow.com/… Thank you for your input. $\endgroup$ – Gideon Apr 25 '13 at 2:15
  • $\begingroup$ Hi @Gideon, no probs, if it was useful could you consider clicking on the "tick" mark next to the answer to accept it, so the question is removed from the "unanswered questions" category. $\endgroup$ – Peter Ellis Apr 28 '13 at 5:56

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.