Browsing by Subject "Gompertz model"

Sort by: Order: Results:

Now showing items 1-2 of 2
  • Ovaskainen, Otso; Tikhonov, Gleb; Dunson, David; Grotan, Vidar; Engen, Steinar; Saether, Bernt-Erik; Abrego, Nerea (2017)
    Estimation of intra- and interspecific interactions from time-series on species-rich communities is challenging due to the high number of potentially interacting species pairs. The previously proposed sparse interactions model overcomes this challenge by assuming that most species pairs do not interact. We propose an alternative model that does not assume that any of the interactions are necessarily zero, but summarizes the influences of individual species by a small number of community-level drivers. The community-level drivers are defined as linear combinations of species abundances, and they may thus represent e.g. the total abundance of all species or the relative proportions of different functional groups. We show with simulated and real data how our approach can be used to compare different hypotheses on community structure. In an empirical example using aquatic microorganisms, the community-level drivers model clearly outperformed the sparse interactions model in predicting independent validation data.
  • Koopman, Jacob J. E.; Kramer, Anneke; van Heemst, Diana; Asberg, Anders; Beuscart, Jean-Baptiste; Buturovic-Ponikvar, Jadranka; Collart, Frederic; Couchoud, Cecile G.; Finne, Patrik; Heaf, James G.; Massy, Ziad A.; De Meester, Johan M. J.; Palsson, Runolfur; Steenkamp, Retha; Traynor, Jamie P.; Jager, Kitty J.; Putter, Hein (2016)
    Purpose: Although a population's senescence rate is classically measured as the increase in mortality rate with age on a logarithmic scale, it may be more accurately measured as the increase on a linear scale. Patients on dialysis, who suffer from accelerated senescence, exhibit a smaller increase in their mortality rate on a logarithmic scale, but a larger increase on a linear scale than patients with a functioning kidney transplant. However, this comparison may be biased by population heterogeneity. Methods: Follow-up data on 323,308 patients on dialysis and 91,679 patients with a functioning kidney transplant were derived from the ERA-EDTA Registry. We measured the increases in their mortality rates using Gompertz frailty models that allow individual variation in this increase. Results: According to these models, the senescence rate measured as the increase in mortality rate on a logarithmic scale was smaller in patients on dialysis, while the senescence rate measured as the increase on a linear scale was larger in patients on dialysis than patients with a functioning kidney transplant. Conclusions: Also when accounting for population heterogeneity, a population's senescence rate is more accurately measured as the increase in mortality rate on a linear scale than a logarithmic scale. (C) 2016 Elsevier Inc. All rights reserved.