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 <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
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