Requesting raw data from previously published research I am working on a mathematical model to help explain some experimental results and generate new hypotheses. Unfortunately, I have neither the resources nor interest to gather experimental data, but in this particular sub-field publishing a model without showing its usefulness on some experimental data is not common.

There are several existing experimental findings that can be explained by my model. However, they are presented in other modeling papers and the raw data is not available with the paper or on the authors' websites. In the papers they only present partially-analyzed data (for instance, they show results averages over participants, but not individual participant's results; or sometimes they only give the results of statistical tests). 

I want to contact the authors for their raw data and have 3 related questions:


What is the protocol for contacting by email to ask for authors' raw data? Is this common? 
Will the researchers expect to be invited on-board as co-authors? Or is a citation to their papers, and an acknowledgement of the form "AK would like to thank X, Y, Z for providing their raw data" sufficient?
If my model (without fits to specific data) is in a pre-print state then should I send a pre-print to the authors I contact? What if the pre-print points out weaknesses in their approach to modeling similar problems?
This will, as it seems nearly all questions on this site, vary based on field. My answer applies to Epidemiology and medical research only. Your mileage may vary.


It is very common for this to happen in my field. There has been an increasing emphasis on using meta-analysis and systematic reviews to summarize bodies of work, and with those there is almost always a need for some more information, raw data, etc. to come from study authors. It's fairly routine now. They may not say "yes" for a number of reasons. One may be that they're working on their own projects in a similar direction. But there are others - that privacy laws prohibit releasing data to just anyone or their funding dictates similarly is a fairly common one. So be prepared for "no".
As for the authorship question, it likely depends on the extent of your data request. Generally, if you're just asking for a few numbers that go into a reported value, then in all likelihood, an acknowledgement is more than sufficient. If, on the other hand, you're asking for access to the raw data from their 5 year, many thousands of dollars cohort study? You're likely going to have a member of their study team be an author on your paper, and there will be far more extensive approval processes than just "Sure, .csv file is attached."
"If my model (without fits to specific data) is in a pre-print state then should I send a pre-print to the authors I contact? What if the pre-print points out weaknesses in their approach to modeling similar problems?" For the initial contact, I'd suggest it isn't necessary, because what you're really trying to find out is "Is the release possible". At later stages, I would expect to see what you were doing, either in a full explanation, or as a pre-print. As to your "What if?" question - so what if it does? Science is about improving the methods we use.