In field studies, somatic production of animals is often calculated by means of the increment summation method, which is based on consecutive samples from the population. The main disadvantage of this method is the lack of any measurement of variability, therefore the statistical significance of the calculated production value is uncertain. This paper shows that in many cases a nonparametric statistical approach called the "bootstrap" can be used to overcome this problem. By means of this procedure, natural variability of production and production to biomass ratios can be assessed by 95% confidence intervals, standard deviation or related parameters from a sample of limited size. © 1990 Springer-Verlag.