This is about the different phases of clinical trials. Here "se abre" refers to starting the process, in my opinion. The sentence would be "The treatment assignment starts once the data analysis study has been completed".

Randomization as a method of experimental control has been extensively used in human clinical trials and other biological experiments. It prevents the selection bias and insures against the accidental bias. It produces the comparable groups and eliminates the source of bias in treatment assignments. Finally, it permits the use of probability theory to express the likelihood of chance as a source for the difference of end outcome.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3136079/


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Note added at 9 hrs (2016-12-24 14:40:03 GMT)
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The treament assignments are opened.

After reading the text again I find it ambiguous. It could mean the above, with "abrir" having the following meaning;
Dar principio a las tareas, ejercicios (RAE).
However, it could also mean that the envelopes containing the treatment assignments are opened, in this case the sentence would mean " The treatment assignment (envelopes) is opened once the data analysis study has been completed.