Another unit commonly used in quantitative paleozoology is biomass. Biomass is defined as the amount of tissue in an area or from a species. It is calculated by estimating an average weight based on similar modern species and multiplying it by the MNI. This yields an estimate of how much the entire population of a species may have weighed. Problems with this measurement include the difference in weight between youngsters and adults, seasonal weight changes due to diet and hibernation, and the difficulty of accurately estimating the weight of a creature with only a skeletal reference. It is also difficult to determine exact age of fossilized matter within a year or a decade, so a biomass might be grossly exaggerated or under exaggerated if the estimated time frame in which the fossils were alive is incorrect.