I estimate forest wood volumes by measuring tree diameters and heights in circular forest plots. From these plots I record the volumes then find the variance of the plot volumes, variance of the mean, and standard error which leads me to a 95% confidence interval for the mean plot volume and total forest volume estimates.

I recently generated histograms of plot volumes for the most recent forest "cruises" and found that there were more reverse-J shaped curves than bell curves. (The histograms display ranges of plot volumes (x) and their frequencies (y)). So the cruises with reverse-J curves show that there is a higher frequence of small plot volumes than large volumes.

My question: Is it OK to generate variance, standard error, and conf interval for non-parametric data? My textbook on forest sampling techniques, while mentioning the actuality of parametric vs. non-parametric data, says nothing about whether variance to confidence interval is appropriate for non-normal data.

Invite everybody or anybody to chime in on this.