I want to compute a regression equation for predicting tree volume by independent variable dbh (diameter at breast height) at 95 % confidence interval for 5% error.

What is the formula for sample size if the total population number is unknown?

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    $\begingroup$ It is highly unlikely you would succeed with this as stated: a prediction concerns trees not in your sample and thereby is subject to the same amount of variation seen in the population, which is going to exceed 5%. But if you want to estimate the mean volume for specified dbh values, you can succeed (because the more data you have, the more certain you can be about the mean). The formula necessarily involves an estimate of the residual variation of the error in the regression of log volume against some function of dbh: do you have such information available? $\endgroup$ – whuber Sep 10 '13 at 14:48

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