Patricia, your point (within-plant variability as a way to weight characters) sounds very interesting. (I am meeting problems of comparing populations which differ in leaf characters that are at the same time variable within a plant - along the leaf spiral). Please, could you send me citation(s) of your papers in this field?
Many thanks! Zdenek Skala [EMAIL PROTECTED] -----Original Message----- From: morphmet [mailto:[EMAIL PROTECTED] Sent: Thursday, October 18, 2007 6:45 PM To: [email protected] Subject: Re: Measure of variability James, Is the degree of the problem going to be proportional to the degree of correlation? Can he ignore a small amount of correlation? Would we agree to avoid anything using a CV and all ANOVA procedures? There are lots of examples out there where these types of things have been done. Be wary, Andrew! Again, I suggest manufacturing a data set with known distributions in order to check that your statistic represents what you want to study. Correlation and non-normality are hard to avoid in plant measures. Soule (1971), who I referenced before, used a statistic much like Anderson's generalized variance, with some additional transformations. I was interested in population variability, but ultimately I became interested in within-plant variability as a way to weight characters. That is where plants, with their modular form, provide a way for us to recognize some kinds of environmental variation. Yrs, Patricia On 10/16/07, morphmet <[EMAIL PROTECTED]> wrote: > > The classical measure for this is the generalized variance which is > the determinant of the covariance matrix. This would probably work > well unless some of the variables were highly correlated. The problem > then is that a perfect correlation results in a generalized variance > equal to zero even there is lots of variability in each variable > studied. > > There are also some ad hoc measures that could be tried such as the > average or geometric mean of the variances. > > The variables should also be in the same units - perhaps > log-transformed morphological measurements. > > ========================= > F. James Rohlf > Distinguished Professor, Stony Brook University > http://life.bio.sunysb.edu/ee/rohlf > > > > -----Original Message----- > > From: morphmet [mailto:[EMAIL PROTECTED] > > Sent: Monday, October 15, 2007 3:46 PM > > To: morphmet > > Subject: Measure of variability > > > > Dear Morphometricians: I am a taxonomist working on a revision of a > > genus with about 80 species of plants (palms). I have a data matrix > > with > > measures of about 20 variables, taken from herbarium specimens. Some > > species are obviously much more variable than others. What I want is > > a > > single measure of variability of each species. What is this? > > > > Thanks. Andrew Henderson > > > > > > -- > > Replies will be sent to the list. > > For more information visit http://www.morphometrics.org > > > > > -- > Replies will be sent to the list. > For more information visit http://www.morphometrics.org > > -- Replies will be sent to the list. For more information visit http://www.morphometrics.org
