Dear professors, Thanks for your prompt reply. However we think there is a little misunderstanding between your replies. According to Dr. Hammer, the classical Iris data set from Fisher provides similar results between the parametric and non-parametric MANOVAS. Dr. Rohlf, on the other hand, suggests that it is expected to have different p-values (in opposite directions of "significance") between the parametric and non-parametric alternatives.
In any case we agree with the suggestion from Dr. Rohlf about the requirements for larger data sets in the non-parametric tests. Given that p-values are not an absolute point of reference we are also relying on complementary descriptions of the distance among samples, and complementary indexes about the robustness of p-values. Pablo Menendez Pablo Jarrin Quoting morphmet <[EMAIL PROTECTED]>: > As Oyvind mentions, the difference in P-values in the two methods is > in > the expected direction. It is difficult to tell whether the magnitude > of > difference is 'reasonable' without knowing sample sizes and the > number > of landmarks. A nonparametric method requires larger sample sizes. > In > the discussions concern was mentioned about the assumption of > normality. > MANOVA is based on the assumption of homogeneity of within-group > covariance matrices. That can be more of a problem that modest > departures from normality (the central limit theorem works for > multivariate data too). > > In the original question it was mentioned that the data shows > differences mostly along the PC1 axis. That implies that the > explanation > for the differences may be one-dimensional. If not size, then > temperature, north-south gradient, etc. In such cases a general > multidimensional method such as MANOVA will have less statistical > power > than a method such as multivariate regression on some possible > explanatory variables. > > ------------------------ > F. James Rohlf, Distinguished Professor > Ecology & Evolution, Stony Brook University > www: http://life.bio.sunysb.edu/ee/rohlf > > > > -----Original Message----- > > From: morphmet [mailto:[EMAIL PROTECTED] > > Sent: Friday, November 09, 2007 9:35 AM > > To: morphmet > > Subject: Re: MANOVA vs npMANOVA > > > > Hi, it is interesting that you get so different > > results between MANOVA and NPMANOVA. Would you > > mind sending me the data set as saved from PAST > > for me to look at? (I'm the author of PAST). > > > > The standard 'Iris' test data set of Fisher gives > > quite similar results between MANOVA and NPMANOVA > > of PAST. In general, the NPMANOVA is expected to > > be less powerful (have higher p values) than MANOVA, > > I believe. > > > > > > Oyvind Hammer > > Natural History Museum > > University of Oslo > > > > > > On Fri, 9 Nov 2007, morphmet wrote: > > > > > Dear colleagues, > > > > > > We would like to take advantage of Dr. Slice's comments about > MANOVA > > to > > > assess differences among species or biological groups. There are > few > > > multivariate normality tests available, however it is often > common > > that > > > Partial Warps (and therefore Relative Warps) from morphological > > > structures do not fit normality. Thus, we think this is the > main > > reason > > > why Dr. Slice recommends the non-parametric alternatives for > MANOVA. > > > More specifically our problem is that we obtain different > results > > when > > > applying either of both parametric and non-parametric MANOVAS. > > > We are at the moment assessing the relationships between > > environmental > > > variables (remote sensing data) and morphological characters > > (geometric > > > morphometrics) in a group of Neotropical bats. Neither of both > types > > of > > > variables fit normality very well. > > > However, what currently puzzle us is the fact that the MANOVA > > performed > > > in SPSS give us significant differences, while the np-MANOVA in > PAST > > > gives us non significant differences. The contrast in > magnitude > > > between both p-values is extreme. We haven?t yet looked at > > additional > > > indexes of overlap, confidence or robustness for p-values. > > > We will be grateful with any comments or suggestions. > > > Pablo Menendez > > > Pablo Jarrin > > > > > > > > > > > > -- > > > 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 > -- Replies will be sent to the list. For more information visit http://www.morphometrics.org
