To me, all the thoughtful suggestions made by our worthy colleagues so far don't quite sound natural ... they don't sound like usual descriptions of a research study design. This is why I'm suggesting the adjective 'sound'.

Here are some of the meanings of this word, from my Webster's Collegiate Dictionary: 'sound' = free from flaw or defect, free from error or fallacy, showing good judgment or sense, exhibiting or based on thorough knowledge. I think the term 'sound' captures the meaning intended by 'inobjetable.'

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Note added at 6 hrs (2008-06-29 07:40:54 GMT)
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A few more example sentences:

" Procedures with animals will avoid or minimize discomfort, distress, and pain to the animals, consistent with sound research design. ..."

" For example, the IRBs evaluate whether procedures to be performed on subjects a) are consistent with sound research design and do not unnecessarily expose ..."

"What is sound research design? How does the University determine if ... "

" Risks to participants are minimized as much as possible, through sound research design and the use of procedures that are not unnecessarily risky."

I would then suggest this translation for the whole sentence:

"The ethics committee has evaluated the study design and considers it to be scientifically sound."

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Note added at 18 hrs (2008-06-29 19:07:15 GMT) Post-grading
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As an afterthought: I would have used some of the other suggestions given here when talking about the _results_ of the research. I think it makes sense to talk about 'unquestionable' or 'indisputable' or 'undeniable' findings or conclusions of a research study.