Where default values are used, uncertainties for emission and removal factors and other parameters are available from GPG2003 (or 2006GL and the Wetlands Supplement), and for Tier 2 and 3 methods will be generated as part of the sampling process. When based on probabilistic sampling, the emissions/removals factors and their uncertainty can be calculated using two broad methods, depending on whether the emission or removal factor corresponds to the difference of carbon densities between strata, or to the change in carbon density of a given stratum over time. The focus of this section is on changes in biomass carbon, non-CO₂ emissions/removals can be calculated analogously if they have also been measured as part of the sampling program.