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A final concern is in understanding the uncertainty in any final synthesis measure. This depends not only on the described factors but also on interdependence of the individual studies contributing to the synthesis (Cramer et al., 2014). For example, in synthesis studies of shifts in the geographic ranges of multiple species it is assumed that each species shifts its range independently of others (e.g., Hockey et al., 2011; Parmesan et al., 2011; Rosenzweig & Neofotis, 2003). In that case the addition of observations of the range shift of an additional species adds substantial new information to the synthesis. However, the independence is hard to confirm when species are shifting their ranges as part of a general relocation of an entire ecosystem: observations for a species that is simply following its food (with the observations of that species already included) will lend confidence to the observations of its food but will not truly add a new item within the synthesis.