[EMAIL PROTECTED] wrote: > Rich Ulrich <[EMAIL PROTECTED]> wrote: > >>Here is some more of what Gene posted -- >>"Of 20 variables measured to account for the variability in species >>richness, total deposition of inorganic N (Ndep, kg N ha?1 y?1) was >>the most important predictor, explaining more than half of the >>variation in the number of species per quadrat (Fig. 2A and Eq. 1).... >>" After accounting for N deposition, mean annual precipitation (MAP, >>mm) explained an additional 8% of variability in species richness. A >>further 5% was explained by the A horizon soil pH (Top pH, Fig. 2B) >>and 3% by altitude (Alt, m). In total, 70% of the variability in >>species richness could be explained by these four variables: ... " >> >> >>Stepwise, I have said before, can give you a shorter list >>of variables when you have a list where everything matters. >> >>Especially, it can give you the *first* variable, if one of them >>stands out from the others. In the above, Deposition does >>account for a huge share of variance; what is unstated (here, >>at least) is whether any of the other (presumably correlated) >>measures were anywhere close to that fraction, univariate. >> >>Stepwise if *famous* for being really lousy at giving you >>the number two and three and four when the relative shares >>of Variance are (for instance) 54, 8, 5, and 3. If they were >>searching for 'explanation' rather than a shorter prediction >>equation, then the authors stumbled badly -- if the stepwise >>result is all they relied on. Again, I have not see the paper, >>so I want my aspersions to be read as being somewhat >>hypothetical, or as being cast against the worst-case scenario. > > > > I think my primary objection to Gene's argument was that he seemed to > suggest that stepwise was superior because it's objective (I hope I did > not misunderstand). It is "objective" in the sense that the machine is > making the decisions, but there is every reason to believe that the > machine is likely not making the _correct_ decisions, so the objectivity > doesn't really help. <Snip> I wasn't making the claim that stepwise was superior. I was trying to understand why the authors specified 'stepwise' as the method if not to convey to the reader that their method was objective In the brief supplementary pdf which is available online for this article, the authors state: 'Stepwise multiple regression was used to create the models between plant species richness (mean number of species per quadrat) and potential environmental drivers (Table S1). Multiple regression analysis assumes that the relationship between X and Y variables is linear, the scale of the variability of the Y values is constant at all values of X, and the errors are independently and normally distributed. These assumptions were examined by residual plots and no violations of the assumptions were found (Wilks-Shapiro statistic, p<0.04, variance inflation factors < 2). We also tested and rejected that the regression was skewed by outliers. The same model resulted regardless of the order in which the variables were entered."
Now to me, the paragraph seems unlikely to be a complete description of the issues involved. The first three potential explanatory variables listed in their Table S1 include: Total Nitrogen deposition (kg N ha^-1 y^-1) Ndep Total deposition NH_3 + NH_4+ (kg N ha^-1 y^-1) N-red Total deposition NO + NO2 + NO3- (kg N ha^-1 y^-1) N-ox This is equivalent to having the variables 1) red+ox 2) red 3) ox In the paper, only total nitrogen deposition makes it into their reported regression equations [Note that Ndep is a prediction from an atmospheric model; it isn't measured at these sites throughout Britain.] I don't see how the 'same model resulted' regardless of the order in which the variables were entered [if N-red and N-ox were entered 1st, Ndep would be unlikely to be selected unless I'm missing a form of N]. Perhaps there is a 3rd form on nitrogen in rain that I'm missing. I don't think urea or other forms of organic nitrogen would be a major factor in rain, but I'm an oceanographer, not an atmospheric chemist. . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
