Ben - Provided that you have a set of objects, for each of which you have recorded the same set of landmarks repeatedly, there are at least two simlpe ways in which you can compare the impact of random measurement error across "true" landmarks [following a Procrustes superimposition if location varies across repeated measures]. 1. In absolute terms: simply compare SD across repeated measures within objects for the actual landmark data (in all dimensions). The smaller the SD's, the better the landmark in terms of being less affected by random ME. 2. In relative terms: calculate the repeatabilities of actual landmark data/locations (in all dimensions). The higher the repeatability, the larger is true variation in landmark location across objects relative to measurement error for that landmark. Does this help?
/Göran Arnqvist At 02:06 PM 7/27/2005, you wrote: > I don't yet see a solution to the repeatability problem. While the >repeatability at each landmark can be assessed at each landmark, the ratio >of the repeatability to the biological variation at each landmark cannot be >calculated using any method I have seen. The work done in this area has >focussed on repeatability/biological-variation of shape variables (Arnqvist >& Martensson, 1998). The ratio is not calculated for any specific >landmarks. The landmarks are the measurements, so how can we know which of >our measurements are good and not-so-good? > Ben >----- Original Message ----- >From: "morphmet" <[EMAIL PROTECTED]> >To: "morphmet" <[email protected]> >Sent: Thursday, July 21, 2005 12:17 PM >Subject: Re: Repeatability > > > > > > I'm a bit late in jumping on the repeatability bandwagon, but for what > > it's worth, regarding semilandmarks, allow me to throw in my ten cents' > > worth. > > > > Their initial positions along the outline are arbitrary, but I assume > > one could probably derive new coordinates for them after having "slid" > > them in TPSRelw, and then treat them as true landmarks for the purposes > > of repeatability. > > > > If one felt compelled to do so. > > > > That would, however, not be statistically valid since their final > > positions will be determined by the positions of those terminal > > landmarks which anchor them. > > > > Repeatability of the semilandmarks will therefore be related to > > repeatability of the terminal landmarks, and my personal feeling is to > > not worry about the semilandmarks too much, but take very special care > > in minimising digitising error in terminal landmarks. > > > > > Lets see, take a x number of individuals, make three repeated > > > measurements, preferably by different people, and estimate the > > > proportion of the variation that is explained by individual versus all > > > the available variation. Either landmark by landmakr, of all at once > > in > > > a MANOVA. > > > > > > For semi landmarks, no idea, never worked with those. > > > > > > Good luck. > > > > > > Kim > > > > > > morphmet wrote: > > > > > >> I was simply wondering what would be the best way, the most > > appropriate > > >> statistical test, to evaluate the repeatability of your landmark > > >> placement. > > >> > > >> Furthermore, would this method also work with semi-landmarks? > > >> > > >> > > >> Thanks > > >> > > >> Martin > > > -- > > > Replies will be sent to the list. > > > For more information visit http://www.morphometrics.org > > > > > > > > > > > > > > > > > > ________________________________________________________________________ > > This message and attachments are subject to a disclaimer. Please refer > > to http://www.it.up.ac.za/documentation/governance/disclaimer/ for full > > details. / Hierdie boodskap en aanhangsels is aan 'n vrywaringsklousule > > onderhewig. Volledige besonderhede is by > > http://www.it.up.ac.za/documentation/governance/disclaimer/ beskikbaar. > > -- > > 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 ******************************************************************** Associate Professor Göran Arnqvist Animal Ecology Department of Ecology and Evolution Evolutionary Biology Centre University of Uppsala Norbyvägen 18d SE - 752 36 Uppsala Sweden Email: [EMAIL PROTECTED] Phone: +46-18-471 2645 Fax: +46-18-471 6484 New! Homepage with complete PDF downloads and more at: http://www.ebc.uu.se/zooeko/GoranA/GA.html ******************************************************************** -- Replies will be sent to the list. For more information visit http://www.morphometrics.org
