Dear Dean and Sonja, Thank you for your suggestions. I am exploring all these potential bias before starting my study. I have learned a lot through all comments and literature suggestions. I deeply appreciated your help.
Best, Anderson On Tue, Jan 9, 2018 at 3:30 AM, Adams, Dean [EEOBS] <dcad...@iastate.edu> wrote: > Anderson, > > > > I don’t think you appreciated the importance of the Murat’s comments on > your earlier post on this same topic. > > > > In theory, there is no problem combining objects digitized at different > magnifications, or even digitized by different researchers. However, before > doing so one must carefully investigate for possible systematic biases in > digitizing, so they may be reduced to the greatest extent possible. If > there is some consistent bias in how objects are digitized in one ‘group’ > relative to the other, this will permeate into perceived differences in > shape that may not exist. A common example with older digitizing tablets > would be differences in digitizing due to the handedness of the person > digitizing. Right-handed and left-handed individuals hold the stylus > differently which can result in consistent perceived shape differences of > the same objects once digitized. > > > > Whether or not you have such an issue with your two magnifications is > unclear. However, it is impossible to evaluate this without additional > replication. Again, as Murat suggested, try digitizing each object multiple > times at each magnification. Then one could obtain estimates of the > variation in digitizing at the same magnification versus across > magnifications to begin to discern whether the between-magnification > variation is greater than one might expect. If it is, then one must dig > deeper to determine why. > > > > I would recommend sorting all of this out before embarking on your > empirical study. Otherwise, interpreting patterns in the final dataset > becomes challenging to say the least. > > > Best, > > > > Dean > > > > Dr. Dean C. Adams > > Professor > > Department of Ecology, Evolution, and Organismal Biology > > Department of Statistics > > Iowa State University > > www.public.iastate.edu/~dcadams/ > > phone: 515-294-3834 <(515)%20294-3834> > > > > *From:* Anderson Feijo [mailto:andefe...@gmail.com] > *Sent:* Friday, December 29, 2017 3:19 AM > *To:* MORPHMET <morphmet@morphometrics.org> > *Subject:* [MORPHMET] Doubt about scalling photos > > > > Hi everyone, > > > > I am starting a new project using GM which I will work with groups with > different sizes (e.g., rodents and small carnivores). I would like to find > a way to use the whole dataset in the analyses, instead of perform set of > analyses for each sized group. So, I did a test using one skull and place > the camera in two different distances to the object (~15 cm and ~30 cm). My > expectation was after scaling (using tpsDig) I wouldn´t have any meaningful > difference. But I got two clear groups that were statistically different. > So, my question is how can I combine 2D landmarks based on photos taken > from different distances of the camera to the object. I have attached here > the tps file (10 copies of the same skull, five at ~15cm and five at > ~30cm). I would be very grateful for any suggestion. > > > > All the best and Happy 2018! > > > > Anderson > > -- > MORPHMET may be accessed via its webpage at http://www.morphometrics.org > --- > You received this message because you are subscribed to the Google Groups > "MORPHMET" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to morphmet+unsubscr...@morphometrics.org. > -- _____________________________________________ Dr. Anderson Feijó Key Laboratory of Zoological Systematics and Evolution Institute of Zoology, Chinese Academy of Science Beichen West Road, Chaoyang District, 100101 Beijing, China Curriculum: *Lattes <http://lattes.cnpq.br/9406413385468571>; ResearchGate <https://www.researchgate.net/profile/Anderson_Feijo>* -- MORPHMET may be accessed via its webpage at http://www.morphometrics.org --- You received this message because you are subscribed to the Google Groups "MORPHMET" group. To unsubscribe from this group and stop receiving emails from it, send an email to morphmet+unsubscr...@morphometrics.org.
