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/<http://www.public.iastate.edu/~dcadams/> phone: 515-294-3834 From: Anderson Feijo [mailto:andefe...@gmail.com] Sent: Friday, December 29, 2017 3:19 AM To: MORPHMET <firstname.lastname@example.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<mailto:morphmet+unsubscr...@morphometrics.org>. -- 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.