Problems in Erickson et al. 2009

نویسنده

  • Nathan P Myhrvold
چکیده

In the course of pursuing research on dinosaur growth rates (Myhrvold, 2013), I attempted to repli-cate the findings of Erickson et al. for Psittacosaurus lujiatunensis (Erickson et al., 2009a). In their study, they use femoral length measurements and age estimates for 80 specimens of P. lujiatunensis to estimate body mass and age, fit a growth curve, and construct a life table. These results were then used to describe certain aspects of developmental timing and to compare with growth in other dinosaurs as well as extant birds and mammals. There are statistical problems with the approach that Erickson et al. use for growth curve fitting (Myhrvold, 2013). This is evident in Figure 3 of their paper, which shows an ordinary linear regression using femur length as the independent variable and age as the dependent variable. Having thereby demonstrated that age is well fit by a linear relationship between femur length and age, Figure 6 of Erickson et al. proceeds to use age as the independent variable and fits a sigmoidal model to the cube of femur length. As discussed at length in Myhrvold (2013), the choice of independent variable in Figure 3 is inconsistent with that in Figure 6. The model is also inconsistent—if there is a linear relationship between age and femur length, then the cube of femur length should fit a cubic curve in age, not a sigmoidal one. Fitting a sigmoidal curve to cubic data gives nonsen-sical results (Myhrvold, 2013). However, the main thrust of this letter is not these choices in statistical methodology, but instead to ask a far simpler question—can the growth curve results of Erickson et al. 2009 be replicated using their stated methods? In Figure 6 of Erickson et al. 2009, they plot a two-parameter logistic growth model, mðtÞ5 ae bt a1e bt 21 , that has been fit to estimated mass-at-age data for P. lujiatunensis. According to the text, " Sigmoidal growth equations were fit using least squares regression and used to describe these data. " These are well known and robust statistical techniques and should be easily replicated. Erickson et al. report the best fit parameters asa537:38 and b50:55. Unfortunately, the published regression equation cannot be replicated—the best fit that I get using the same function specified in the paper is radically different with a 5 88.4 and b 5 0.345 (for the full data set). Erickson et al. state that they …

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عنوان ژورنال:

دوره 298  شماره 

صفحات  -

تاریخ انتشار 2015